Showing results for 
Search instead for 
Did you mean: 

CUCM One-way audio and MTP Problems


I am experiencing intermittent one-way audio problems and could really use some help!.  

We are using CUCM 10.5.1 with a SIP trunk to an external SIP provider.  Internal calls are bi-directional and working fine.

'MTP is required' is checked on SIP trunk.

The one-way audio is not always happening, its intermittent.  During normal working calls via the external SIP Provider, all RTP packets are being sent from IP phone to CUCM, which forwards it to the SIP provider.  

When we have one-way audio issues, the RTP media packets are being sent directly to the SIP provider from the IP Phone.  There is no network access from IP phone to SIP provider which will be why we only have one-way audio.

Network access should not be the cause because audio should be going via CUCM and not direct from IP phone to external SIP provider.

Any advice will be appreciated!  Many thanks!

Cheers, Mark


Hi Mark, 

Hi Mark, 

If MTP is checked on the SIP trunk on your end, it should be checked on the service provider SIP trunk too, make that change, reset the trunk, do a test call and let me know. 



Cisco Employee

Ideally this needs to be

Ideally this needs to be checked via SIP signaling in the callmanager service traces from cucm or via debug ccsip messages from gateway for a good and bad call to understand why MTP goes out of picture even when MTP required is enabled.



Hi.  The issue has come back

Hi.  The issue has come back today and lots of people are having one-way audio issues.

I did a trace on CUCM and an IP Phone.  The two attachments are snippets of a trace from CUCM to the same IP phone.  I can see a difference in the SDP packet between a good and bad call.  

As you can see from the attachments, CUCM is telling my desk phone to send media to the CUCM IP in a good call, but in a bad call, its providing the address of our external SIP provider.  All the media to the SIP trunk should go via CUCM and never direct from ip phone to sip provider.

Any ideas where I need to look to figure why sometimes CUCM is giving the wrong address out to the IP phone?

VIP Rising star

Check for CUCM SDI Logs to

Check for CUCM SDI Logs to find out if MTP Allocation is failing.

You should find message similar to 

SendMTPResourceErrToSender - ERROR  AllocateMtpResourceReq failed

We and further investigate based on the SDI Logs.

Please Rate and mark correct If applicable




Thanks for the advice guys.

Thanks for the advice guys.  I ended up rebooting the server and the problem went away.  I still don't really understand what caused the issue.  Its the second time in 1 month that it happened and the reboot seems to clear the issue.

I will email the service provider to ask if they have MTP required at their end, its an Asterisk system but maybe that has something similar.

If it happens again I will capture a trace and see if I can learn a bit more about the cause.

Thanks :-)


You might want to reset the

You might want to reset the SIP trunk next time.Often fixes things.

Rising star

Just wondering if you have

Just wondering if you have any of the following:

1. All hardware based MTP configured in the MRG and MRGL?

2. Even if Software based MTPs are configured they are in an MRG / MRGL that is not getting assigned to the phones?

3. If you have Transcoder listed Higher in the MRG and then the MTP, that will make all the Transcoders (Transcoders can act as MTPs as well) engaged and starving any MTPs for  new calls


Please rate all helpful posts.


Re: CUCM One-way audio and MTP Problems

My case i s similar but our situation is that it is on ISDN PRA line , where after a call transfer one way voice is encountered , external caller can hear but ip phones user cannot hear .

please  advise as all set up in Media resources are well configured. 

Cisco Employee

Re: CUCM One-way audio and MTP Problems

Hi Saeed,


This could be due to wrong audio port, IP being shared or may be the Media resources not getting allocated. It can be confirmed from CCM Logs and debug from the gateway. Share the CCM logs with call details and debug from the gateway. 



Prashant Sharma


Rate if you find this helpful.