One-way audio with analog trunk


I’m not on-site yet, but as it’s reported to me, all of the phones (GXP2170 and GXP1630) periodically have a one-way audio issue when using an analog trunk. They have no SIP trunk, only one analog trunk with 3 ports.

I’m very familiar with one-way audio through routers due to NAT issues, but there are no SIP trunks here and all phones are on the same LAN as the UCM.

The problem is inconsistent, but they claim that rebooting the UCM fixes it for several hours. This system had been up and running for 300 days without a problem. We upgraded the firmware anyway and the problems remains. I had a tech swap out the UCM for a new one, restored a backup, and the problem returned a few hours later.

I’ve asked them to try internal calls next time the problem happens to help rule out handsets and network issues. I’m not entirely convinced that it isn’t a Telco problem, but have not been able to reproduce the problem on a butt phone yet.

Any other thoughts? Thank you!


First make sure that the problem is not directly on the PSTN (even if I’ve never seen a PSTN line with one-way audio), plug in an analog phone directly and try.


The NAT issue is not necessarily just related to the trunk aspect. The phones may also play a part in how the PBX handles calls. You did not mention which side can hear and which can not. You also did not mention if the issue is only on calls made, calls taken or both. You did mention if the extensions are allowed to direct media or if not in which case the PBX always handles. You did not mention which firmware on the UCM or phones.

Regardless of what type of connectivity for telephony is used, you should still set the PBX up for NAT. This entails the settings in PBX Settings, SIP Settings, NAT and ToS. Identifying the local network on the NAT settings page is certainly one key aspect.

You might consider breaking out the 3 PSTN lines into separate trunks. If there is an analog line issue, perhaps this will help isolate which ones as presumably when a call has an issue you can see which trunk was used in the associated CDR. Give each new trunk a prepended name to ease the identity. You will need to add rules to accommodate.

It would really be nice to get a capture of a call with the issue.