UCM6208/GXP2160 Dropping Calls when answered, then service unavailable showing on screen


As the title says, I have a fairly new installation of a Grandstream UCM6208 with GXP2160 phones. I have one user that has had an issue pop up today (phones have been in use for about two weeks now) where when the user attempts to answer some calls they drop after a second of silence, then “Service Unavailable” is shown on their phone.

I have no idea what could be causing this, outbound calls and inbound calls answered by other users are not having issues. When I look at the extensions on the UCM web portal that extension shows as available and I am able to ping it and log on as admin on the extension’s web portal.

Has anyone else had a similar issue?


missing scenario, details and exact problem, to be able to answer you


What all details do you need?

Currently calls come in through an analog port into a call queue that has three users in it. If the call is not answered after 30 seconds it goes to a ring group. The user having the issue is in the original call queue. The phone rings, shows up on caller id, she presses the green “Answer” soft key (thus answering the phone on speaker), nothing happens for about 5 seconds, then the call drops. After the call ends, the words “Service Unavailable” are displayed on the screen.


for example:
analog lines or SIP trunks
remote or local extensions
fw of UCM and the phones used
UCM behind NAT?
scenario on incoming and outgoing calls
these are things that the reader cannot know :wink:


Analog lines - we have six. Any additional details needed on them?
All extensions local.
UCM version 1.6A
Version information: Boot

Phone Information:


UCM is not behind NAT
Scenario is as described in previous reply.

There are 27 extensions, and as far as I can tell they are all set up the same general way (obviously account name and password are different, as well as speed dial buttons). The extensions are connected to two Netgear Managed PoE Switches (Basic setup, so nothing blocking traffic. I’ve checked the switches, no errors or dropped or blocked packets). LAN port of UCM, switches, phones are all on the same network with no ethernet connection to anything else. UCM is DHCP on this network, each phone is mapped to its assigned IP address. WAN port on UCM is on local office network (with internet through router).


FW UCM to be updated, for safety, but this may not address the issue.

So if you have only one PSTN line do the following:
1 - disconnect the line from UCM, connect it to an analog telephone and make several incoming and outgoing calls, just to understand if the problem is already present on the line or not.
2 - reconnect the line to UCM and make sure you set the CLIp recognition, impedance and regionalization correctly (you can find everything in the UCM manual and you will find many posts about it on the forum). Restart UCM.
3 - activate a new FXS extension (for test only) sets the incoming call of the PSTN line only as the destination of the FXS extension created.
4 - do several tests in and out of created FXS. IS THE PROBLEM PRESENT OR IS EVERYTHING WORKING REGULARLY?
5 - if you make several local tx / rx calls between GXP2160 phones everything works normally?
6 - disconnect all GXP2160 phones and connect only one GXP2160 phone directly to UCM (then delete any SWITCH or other network tools).
7 - on UCM it routes incoming calls only to GXP2160 which is running. Make several tests on entry and exit.
8 - IS EVERYTHING NOW WORKING REGULARLY? if the answer is yes, the problem must be found in the IVR / CODA set incorrectly, if instead you continue to have problems the problem must be found in the list in number 2 of my list.