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Cisco Dropped Calls fix January 22, 2010

Posted by jamesisaac in Uncategorized.
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All of my time has been occupied recently with a pretty significant upgrade of Cisco Call Manager from 3.3 to 7.1, skipping all of the in-between steps. Due to hardware EOL and new requirements, we purchased five new servers for our various bits and pieces. This gave us the advantage of building our new system side-by-side, dropping it in on a segmented network, and actually being able to test our call routing by having our telco provide a test number on the new PRI. The actual change-over simply involved calling the telco and having them switch our number from one circuit to the other, and us unplugging the old master Call Manager server from the network and directing phones to the new Call Manager server.

One issue that we did run into post-install had to do with dropped calls. With the way our system is designed, our phones are in our offices while the PRI itself is at the data center. Thus any call quality issues have to be debugged through several routers and switches.

The symptoms were hard to pin down – we got reports that calls were suddenly disconnecting with a fast busy. We looked at QoS stats, line utilization, PRI slips, and all the while tried to reproduce the errors. Finally we did by a happy accident. We had two calls conferenced together through the outside lines, both on speakerphone. After playing with the echo and delay for a bit, we muted both phones – and within 60 seconds one failed with a fast busy. The lightbulb went on! After making a few more test calls, we realized that the calls drop when both phones are muted or both parties are silent for a period of time. Some googling brought this result: 


And the fix, clipped from the page:

parshah 139 posts since

Sep 19, 2007 3. Re: Call Manager – Call Disconnect Jun 8, 2009 10:16 AM


For the disconnect issue with external calls, looks like either the GW or PSTN disconnects

the call when there is silence in the RTP.

Can you check if you have

no mgcp timer receive-rtcp

on the MGCP gateway. If not, can you add it and see if that helps.

And in fact, that was the solution. We added the “no mgcp timer receive-rtcp” on our PRI gateway router, and that fixed the dropped calls.