Routing a CUCM Extension Through 3CX to Existing VoIP Provider / SIP Trunk

[syadmins]

Hello 3CX Community,

We have a mixed 3CX/Cisco CUCM environment and would like to confirm whether 3CX can support the call-routing setup below without using CFD or direct call-forwarding workarounds.

Current Environment​

• 3CX: v20 Update 9
• Cisco CUCM: v15
• Cisco IP Phone: Extension 1013
• Public DID / External Caller ID for 1013: 3203256
• 3CX and CUCM are already connected using a Generic SIP Trunk
• The VoIP provider SIP trunk is currently connected to 3CX
• DID 3203256 belongs to the same VoIP provider account/SIP trunk used by our existing 3CX numbers

The Cisco phone must remain registered exclusively to CUCM. We do not want extension 1013 or the Cisco phone registered to 3CX.

Intended Call Flow​

Inbound:

VoIP Provider
|
| DID 3203256
3CX
^
|
| Generic SIP Trunk
v
CUCM
|
v
Cisco Phone - Extension 1013


Outbound:

Cisco Phone - Extension 1013
|
v
CUCM
|
| Generic SIP Trunk
v
3CX
|
v
VoIP Provider
For outbound calls from extension 1013, the external caller ID should be presented as 3203256.

For inbound calls to 3203256, 3CX should route that DID specifically through the SIP trunk toward CUCM, where CUCM will then route it to extension 1013.

All other DIDs on the provider trunk should continue to be processed normally by 3CX.

Why the Provider Is Connected Through 3CX​

Our 3CX server has two network interfaces:

1. One interface for our normal internal network and 3CX management access.
2. Another interface providing Layer 3 connectivity toward the VoIP provider network.

We considered connecting CUCM directly to the provider network as well, but our CUCM deployment cannot be given the additional network interface/connectivity required for this setup, and changing the existing CUCM IP/network configuration is not an option.

For that reason, we would like 3CX to act as the intermediary between CUCM and the VoIP provider.

Question​

Can this be accomplished using standard 3CX inbound/outbound routing and the existing Generic SIP Trunk between 3CX and CUCM, without using CFD?

Ideally, we are looking for something along the lines of:

• Incoming DID 3203256 → route to the CUCM SIP trunk
• CUCM receives the called number and routes it to extension 1013
• Calls from CUCM/extension 1013 → route through 3CX to the VoIP provider
• 3CX presents 3203256 as the outbound caller ID
• No direct registration of extension 1013 to 3CX
• No CFD application or call-forwarding workaround

We found some examples where people achieved similar behavior using CFD or by forwarding calls directly, but we would prefer to avoid that approach if normal SIP routing can handle it.

Is this topology supported/recommended in 3CX v20, and if so, what would be the correct way to configure the inbound and outbound routing between the provider trunk and the CUCM Generic SIP Trunk?

Thank you.