I recently ran into an unusual problem with One Way Audio and Unity Connections this past week. Basically Employees were complaining that when Customers were calling in over the PSTN, and attempting to leave a voicemail message they were not hearing the personal greeting, but instead silence. I was initially convinced that the problem lied with either Unity Connections or the individual voicemail boxes.
I’ve run into issues before where a user might have a large number of voicemails which would sometimes cause strange and unexpected behavior so I started off by asking a couple of the them to delete any unecessary messages, and re-record their personal greetings. When that didn’t work I deiced to delete one of their entire voicemail boxes all together.
Initial Troubleshooting
- Deleted and re-record personal voicemail greetings
- Deleted at least one entire voicemail box
- Verified the Branch Regional codec settings
- Verified the Branch Media Resource Group settings
- Verified the Branch MRG was a member of the correct Media Resource Group List
- Verified that the Common Device Configuration was assigned to the Phones
As is it turned out none of the above actually helped to resolve the issue, so I decided to start investigating the possibility of a routing issue. In most cases the common cause of one way audio is usually due to some type of asymmetrical routing issue. I had a strong feeling that this wasn’t the issue, but I to check regardless.
I started off by simply checking to make sure that the voice network was being advertised via BGP upstream to my Service Provider MPLS PE router. I also checked the far end to make sure the prefix was in the routing table.
Router#show ip bgp neighbor 61.73.34.229 advertised-routes
BGP table version is 8187, local router ID is 63.73.34.230
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, x best-external
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 172.17.249.0/24 0.0.0.0 0 32768 i
*> 192.168.249.0 0.0.0.0 0 32768 i
Total number of prefixes 2
In my environment all voice related traffic runs over the 192.168.x.x prefixes so you can clearly see that the 192.168.249.0 prefix is being successfully advertised upstream to it’s PE neighbor in this case 63.151.34.229. If the prefix wasn’t being advertised I would be facing other issue beyond one way audio such as phones not registering with Call Manager among a lot of other things.
At this point the only thing I really know for sure at this point with the one way audio was that it was isolated to this one branch location and all the employees voicemail behind this particular remote branch office PSTN. So I decided to start focusing my efforts on the gateway itself by comparing it with other locations that were not experiencing the same issue.
After comparing the configuration from a known good gateway I finally discovered that the MGCP portion of the configuration was missing the media and control bindings.
Router#show running-config | section mgcp
mgcp
mgcp call-agent 192.168.0.10 2427 service-type mgcp version 0.1
mgcp rtp unreachable timeout 1000 action notify
mgcp modem passthrough voip mode nse
mgcp package-capability rtp-package
mgcp package-capability sst-package
mgcp package-capability pre-package
mgcp profile default
Router#
After comparing countless lines of configuration between a location that was not experiecning the one way audio issue I finally discovered the problem. As you can see from above I didn’t configure the necessary bindings for the MGCP media and control on the voice sub interface.
Router#
Router#configure terminal
Router(config)#mgcp bind control source-interface GigabitEthernet0/0.2
Router(config)#mgcp bind media source-interface GigabitEthernet0/0.2
Router(config)#no mgcp
Router(config)#mgcp
Router(config)#end
Router#
Turns out that the lack of media and control bindings for MGCP was causing the one way audio issues when calls were placed over the PSTN. Unfortunately it took me a couple of days to determine the root cause of of this strange behavior. Originally I was focused on the codec’s that were being negotiated through the PSTN gateway. This type of behavior will also occur when someone has forwarded their phone lets say to their Cell phone. For example if someone has their phone forwarded and you call their extension internally you user placing the call will experience one way audio.
I hope you found this post on One Way Audio and informative. Be sure to let me know what you think by leaving suggestions, and feedback in the comments section below. You can find out more about these and other articles be checking out recent posts and archives. To learn more about me be sure to check out the About page. And as always thanks again for visiting The Packet.
