Call forwarding from Cisco Phone to External number OK but no Audio both ways

[Farhad Ashna]

Hi All,

We currently have the below call flow configured in our IPTEL environment.

Phone(SCCP) -> CUCM -> SIP Trunk -> Gateway(ISDN) -> ISDN Provider

When we attempt call forwarding on an internal extension to both personal mobile and business mobile there is no audio both ways. When there is an incoming call ringing on the mobile, I answer, I cannot hear the caller and the caller cannot hear me. (This seems to be an intermittent problem, but more calls are missed than received). This issue only occurs when call forwarding to an external number.

Can someone please advise what could be causing this and why is it intermittent?

Thanks

[Shadi Shami]

Hello,

On the SIP trunk, can you the MTP required check box checked?

Otherwise, we need to check CCM and GW logs.

Thank you,

Shadi

[Amit Sharawat]

Hi Farhad,

This is a case of Hair-pinning call. The TDM voice will come to the gateway and the DSP will process it and will convert it IP packet stream, and it will convert is again back to TDM voice and will send out the "forwarded" number on the different B-channel.

Please collect PCM capture for a failed call on both the B-channels: 1) the one from which the call came. 2) the one through which call went out to forwarded number.

Here is the link to gather PCM capture. Make sure that you check you IOS version, the commands are different after 5.2(2)T1 and later:

https://supportforums.cisco.com/document/53736/how-enable-pcm-capture-isr-voice-gw

~Amit

[Farhad Ashna]

Hi Amit,

Do you know why the issue will be intermittent as it is working to certain mobile phones?

Regards,

Farhad

[Farhad Ashna]

Hi Shadi,

The MTP required is not checked however as the issue is intermittent hence I do not think MTP required box check will resolve it.

I think this could be due to the ports not being negotiated correctly or the handover of the calls are not taking place in the correct manner.

Regards,

Farhad