How to Configure External Call Forward in CUCM

CUCM External Call Forward allows calls received on a Cisco IP phone extension to be forwarded to an outside telephone number, such as a mobile phone, landline or PSTN destination.

For example, an employee has Cisco IP phone extension 2001 and wants calls to be forwarded to a mobile number when they are away from the office.

The basic call flow is:

Caller → CUCM → Extension → External Call Forward → PSTN/SIP Trunk → Mobile

Cisco Unified Communications Manager supports external destinations for call forwarding, provided the appropriate Calling Search Space (CSS) allows CUCM to route the forwarded call.

What Is CUCM External Call Forward?

CUCM External Call Forward redirects a call from a Cisco IP phone directory number to an external telephone number.

Common examples include:

• Forwarding an office phone to a mobile phone
• Forwarding calls to a home office
• Forwarding calls to another external business number
• Forwarding unanswered calls to a mobile
• Forwarding busy calls to an external number
• Forwarding all calls to a PSTN destination

For example:

Cisco IP Phone
Extension: 2001

        ↓

CUCM

        ↓

External Call Forward

        ↓

0501234567

        ↓

SIP Trunk / Voice Gateway

        ↓

PSTN

The external number must be reachable through the CUCM dial plan.

What You Need Before Configuring External Call Forward

Before configuring external call forwarding, make sure you have:

• Cisco Unified Communications Manager
• A registered Cisco IP phone
• A directory number
• A working PSTN or SIP trunk
• An appropriate external route pattern
• A Calling Search Space (CSS)
• The external destination number

You should also confirm the number format required by your SIP provider or PSTN carrier.

For example, your users may dial:

0501234567

while the carrier may require:

+971501234567

The CUCM dial plan must account for this difference.

Step 1: Open the Cisco IP Phone Configuration

Log in to Cisco Unified CM Administration.

Go to:

Device → Phone

Find the Cisco IP phone that you want to configure.

Open the phone configuration and locate the required Directory Number.

For example:

Directory Number: 2001

The Call Forward settings are configured for the directory number.

Step 2: Find the Call Forward Settings

Within the directory number configuration, locate the Call Forward settings.

Depending on the CUCM version and configuration, you may see options such as:

• Forward All
• Forward Busy
• Forward No Answer
• Forward No Coverage
• Internal destination
• External destination
• Calling Search Space

Cisco documents separate external destination and CSS fields for several forwarding conditions.

Step 3: Configure Forward All External

If you want every call to the extension to immediately forward to a mobile or PSTN number, configure Forward All.

For example:

Forward All External Destination:
0501234567

Then select the appropriate:

Forward All CSS:
CSS-External-Forward

The exact field labels can vary depending on the CUCM release.

Cisco explains that when Call Forward All is activated, CUCM validates the forwarding destination using the configured CFA CSS settings.

Example

Suppose:

Extension:
2001

Forward All External:
0501234567

Forward All CSS:
CSS-External-Forward

The call flow becomes:

Caller
   ↓
Extension 2001
   ↓
Forward All
   ↓
0501234567
   ↓
PSTN Route Pattern
   ↓
SIP Trunk
   ↓
Mobile

Step 4: Configure Forward No Answer External

If you want the Cisco IP phone to ring first and then forward the call when nobody answers, use Forward No Answer External.

For example:

Forward No Answer External Destination:
0501234567

Forward No Answer External CSS:
CSS-External-Forward

The call flow becomes:

Caller
   ↓
Extension 2001
   ↓
Phone rings
   ↓
No Answer
   ↓
0501234567
   ↓
PSTN

Cisco specifically documents Forward No Answer External Destination and Forward No Answer External Calling Search Space for this purpose.

Step 5: Configure Forward Busy External

You can also forward calls when the line is busy.

For example:

Forward Busy External Destination:
0501234567

Forward Busy External CSS:
CSS-External-Forward

The call flow becomes:

Caller
   ↓
Extension 2001
   ↓
Line Busy
   ↓
0501234567

Cisco documents separate Forward Busy External destination and CSS settings for external calls.

Step 6: Configure the Calling Search Space

This is one of the most important steps when configuring CUCM External Call Forward.

The Calling Search Space determines which partitions CUCM can search when routing the forwarded call.

For example:

CSS-External-Forward

        ↓

PT-PSTN

The PSTN route patterns must exist in a partition accessible by this CSS.

Cisco explains that CSSs are ordered lists of partitions that determine which route-plan partitions a calling device or forwarding operation can access.

Example CSS Structure

A simple configuration could look like:

CSS-External-Forward
        |
        +--- PT-Internal
        |
        +--- PT-PSTN

The CSS can then access both internal and PSTN destinations.

Step 7: Check the PSTN Route Pattern

The external destination must match a valid CUCM route pattern.

For example:

05XXXXXXXX

or another pattern appropriate for your organization’s numbering plan.

Do not copy this example directly into a production system without checking your actual UAE numbering plan and carrier requirements.

The route pattern should ultimately point toward the correct route list, route group and SIP trunk or voice gateway.

Step 8: Check the Route List

A typical outbound configuration looks like:

External Number
       ↓
Route Pattern
       ↓
Route List
       ↓
Route Group
       ↓
SIP Trunk
       ↓
PSTN

For example:

RP-PSTN
   ↓
RL-PSTN
   ↓
RG-PSTN
   ↓
SIP-TRUNK

This is what allows CUCM to send the forwarded call toward the external network.

Step 9: Save and Apply the Configuration

After configuring the destination and CSS:

1. Click Save.

2. Click Apply Config if required.

3. Confirm that the Cisco IP phone remains registered.

4. Test the forwarding function.

Cisco’s documentation indicates that the forwarding destination is validated using the applicable Call Forward CSS.

Step 10: Test CUCM External Call Forward

Use another phone to call the extension.

For example:

Test Call
    ↓
Extension 2001
    ↓
External Forward
    ↓
0501234567

Verify:

• The original phone rings when expected.
• The call forwards at the correct time.
• The mobile phone receives the call.
• The caller ID is correct.
• Two-way audio works.
• The call disconnects normally.

Forward All vs Forward No Answer vs Forward Busy

Understanding the three common options is important.

Feature	What it does
Forward All	Immediately forwards calls
Forward No Answer	Forwards after the phone does not answer
Forward Busy	Forwards when the line is busy

Forward All

Use this when the employee wants all calls to go directly to another number.

Caller
 ↓
Extension
 ↓
Mobile

Forward No Answer

Use this when the employee wants the desk phone to ring first.

Caller
 ↓
Desk Phone
 ↓
No Answer
 ↓
Mobile

Forward Busy

Use this when another call is already active.

Caller
 ↓
Extension Busy
 ↓
Mobile

How to Forward a CUCM Phone to a Mobile Number

A common example is forwarding extension 2001 to:

0501234567

Configure the external forwarding destination and assign a CSS that can access the PSTN route pattern.

Conceptually:

Extension:
2001

External Destination:
0501234567

External CSS:
CSS-External-Forward

Then:

CSS-External-Forward
        ↓
PT-PSTN
        ↓
05XXXXXXXX
        ↓
SIP Trunk
        ↓
Mobile Network

Why CUCM External Call Forward Does Not Work

If you configured the external number but forwarding fails, the most common problem is the Calling Search Space.

For example:

Destination:
0501234567

CSS:
CSS-Internal

If CSS-Internal cannot access the PSTN partition, CUCM cannot route the forwarded call.

Cisco notes that if partitions and CSSs are being used, the appropriate Call Forward CSS should be configured; leaving the relevant CSS as None can cause forwarding to fail when the destination is in a partition.

Common CUCM External Call Forward Problems

Problem 1: Forwarding does not activate

Check:

• External destination
• Call Forward CSS
• Partition
• Route pattern
• Number format

Problem 2: Phone forwards internally but not externally

This usually indicates that the CSS used for forwarding can access internal destinations but cannot access the PSTN partition.

Check:

CSS-External-Forward
        ↓
PT-PSTN

Problem 3: Call reaches the wrong number

Check:

• Route pattern
• Translation patterns
• Digit manipulation
• Prefix digits
• Number normalization

Problem 4: Call reaches mobile but caller ID is wrong

Check:

• Calling-party transformation
• External Phone Number Mask
• SIP trunk configuration
• Carrier caller-ID requirements

Problem 5: Call is rejected by the carrier

Check whether the carrier expects:

0501234567

or:

+971501234567

or another approved format.

The CUCM dial plan may need to transform the number before sending it to the provider.

How to Troubleshoot External Call Forward

Follow this sequence:

1. Check the forwarding destination

Make sure the number is correct.

2. Check the external CSS

Verify that the Call Forward CSS can access the required PSTN partition.

3. Check the route pattern

Confirm that the destination matches a valid route pattern.

4. Check the route list

Make sure the route pattern points to the correct route list.

5. Check the SIP trunk

Verify that the SIP trunk is operational.

6. Check number formatting

Confirm that the called and calling numbers are in the format expected by the carrier.

7. Use Dialed Number Analyzer

CUCM Dialed Number Analyzer (DNA) can help determine which partitions, CSSs, translation patterns and route patterns are involved in routing a number.

CUCM External Call Forward Security

External forwarding should be controlled carefully.

If users can forward calls to unrestricted external numbers, the CUCM system may generate unwanted PSTN traffic or expose the organization to toll-fraud risks.

Consider:

• Restricting international destinations
• Creating a dedicated external-forward CSS
• Limiting access to expensive routes
• Monitoring unusual call activity
• Separating internal and external partitions
• Reviewing forwarding permissions regularly

A dedicated CSS makes it easier to control exactly which destinations external forwarding can reach.

Example Complete Configuration

Here is a simplified design:

Directory Number:
2001

Forward All External Destination:
0501234567

Forward All CSS:
CSS-External-Forward

CSS-External-Forward:
PT-PSTN

PT-PSTN:
05XXXXXXXX

Route Pattern:
05XXXXXXXX

Route List:
RL-PSTN

Route Group:
RG-PSTN

Gateway:
SIP Trunk

The resulting call flow is:

Incoming Call
      ↓
Cisco IP Phone 2001
      ↓
Forward All External
      ↓
0501234567
      ↓
CSS-External-Forward
      ↓
PSTN Route Pattern
      ↓
Route List
      ↓
SIP Trunk
      ↓
Mobile

This is a conceptual configuration. Your actual CUCM configuration will depend on your numbering plan, carrier, SIP trunk and existing dial plan.

Frequently Asked Questions

How do I configure external call forwarding in CUCM?

Open the phone’s directory number configuration in Cisco Unified CM Administration, configure the appropriate external Call Forward destination and assign a Calling Search Space that can route to the PSTN destination.

How do I forward a Cisco IP phone to a mobile?

Configure Forward All External, Forward No Answer External, or Forward Busy External, depending on the desired behavior. Enter the mobile number and select a CSS that can reach the PSTN route pattern.

Why does CUCM external call forwarding fail?

Check the Call Forward CSS first. The CSS must have access to the partition containing the route pattern for the external number. Cisco specifically recommends configuring Call Forward CSS when partitions are in use.

What is the difference between Forward All and Forward No Answer?

Forward All immediately redirects calls. Forward No Answer allows the phone to ring and forwards the call only when the configured no-answer condition is reached.

Can CUCM forward calls to a mobile phone?

Yes. Cisco documents external destinations as valid call-forward destinations unless restricted by the system configuration.

Can I restrict external call forwarding?

Yes. Use partitions and Calling Search Spaces to control which external route patterns the Call Forward operation can access.

Conclusion

Configuring CUCM External Call Forward is straightforward once the relationship between the directory number, forwarding destination, CSS and PSTN route is understood.

The basic process is:

Cisco IP Phone → CUCM Directory Number → External Call Forward → CSS → Route Pattern → Route List → SIP Trunk/Voice Gateway → PSTN

If forwarding does not work, start with the external destination and Call Forward CSS, then check the PSTN route pattern, route list, SIP trunk and number formatting.

For Cisco CUCM deployments in the UAE, always verify the required mobile, landline and international number formats with your SIP or PSTN provider before implementing the final production dial plan.

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