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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!


9 Replies 9

Clifford McGlamry

I've seen this one before.  Did you bind the H323 against the same interface that you have listed in CUCM as the H323 gateway?


Posting your config would be helpful.  

We use only Gigabit Ethernet 0/0/0 of the Router. But yes i did a h323-bind on the interface.

So what's the second leg here. A pri?

I have to correct me. One customer has a bind h323 and the other does not have a bind.


Just a thought of mine but do the commands "allow connections sip to h323" and the other way around have to be entered in the "voice service voip"?

Yes, if you want to allow those combinations they do.

So looking back over the thread, you don't need those. 


Check your basic plumbing and make sure that packets from the phones can get to the gateway.


the strange thing is that some calls work and some calls not. Same calling and called party. 

This installations work for over more than 6 years without any problem. After the change of the Voice Gateways (ISR Router), we have the problems. So packets from phone to gateway are OK.

The second call leg to PSTN is a PRI.

Level 1
Level 1



Did you ever fix this ?  I have the same one way voice problem going from H.323 or SIP to ISDN (BRI) when a 4351 is upgraded from 15.5-2.S6 to 16.9.4.  have ended up downgrading back and two way voice then works fine.  SIP to SIP through the same router works fine across both versions of IOS.

No! But i didn‘t try with brand new versions yet. Let me know if you fix it 😁