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Cisco 4321 ISR One-Way Voice

Level 1
Level 1

Hello, folks,
I have two customers with Cisco 4321 ISR routers. We have exactly the same problem with both customers.
There is (not always) one-way voice for outgoing calls. The other party does not understand the internal subscriber. The other direction works. It is not a single telephone but a problem covering the whole area. We could already find out that the Cisco 4321 routers are responsible in some direction. Both customers exchanged Cisco 2900 routers for these 4321 routers. After the problem could not be solved quickly, the old 2900 routers were installed again. The problem disappeared immediately!

I already found in the release notes and a blog that the 4000 routers use a different RTP port range. For this reason we have adapted the RTP ports in the "voice service voip" accordingly. But also without success. We also have updated IOS Image to the latest 16.x Release. CUCM is also on latest 11.5.x.


Here is the setup:


Customer One
ISDN BRI -> ISR 4321 -> H323 -> CUCM 11.5 -> IP Phone


Customer Two
ISDN PRI E1 -> ISR 4321 -> H323 -> CUCM 11.5 -> IP Phone


Problem exist in IP Phone to PSTN. External callers did not hear RTP.


Has anyone seen this issue? How can we troubleshoot this? We can exclude the network infrastructure that everything has already been moved to a subnet (without success). There are also no firewalls included.


Any help is welcome!

Thanks a lot!


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