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SIP trunk - RTP source port incorrect


Hoping for some guidance with an issue...!

We have a CUCM 7.1 system and have setup a SIP trunk to a third party voice gateway. The voice gateway will terminate an internet SIP trunk and pass calls into the CUCM system. We have been doing some testing and can make and recive calls fine to and from the voice gateway.

We also have a voicemail system that runs on a Windows 2003 server that connects to CUCM using the Cisco TSP software install.

When we divert an incoming call from the voice gateway to the voicemail system we get one way audio...

When reviewing a packet caputure (from the gateway) we can see the RTP audio to and from the voicemail system but the gateway is ignoring the traffic. After a bit of digging this is because the voicemail system is using an incorrect source port.

in the SIP headers we see the gateway specifiy its RTP port (16398) and then later in the trace we see the Cisco give back the IP of the Voicemail system and it's port (20012).

The gateway proceedes to talk to the voicemail system using src 16398 to dest 20012

The voicemail replies from src 3852 to dst 16398 and so this traffic stream is ignored!

Anyone have any simliar experiences or ideas about how to make the Cisco TSP install use the correct source port?


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New Member

Re: SIP trunk - RTP source port incorrect

Provide CCM Service detailed traces please.Best way to see what is happening...

Sent from Cisco Technical Support iPad App

Regards, Avinash
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