Incoming Calls Cannot Be Answered Intermittenly... FreePBX with SIP Trunks

This is a strange and random problem. Starting about a week ago an occasional incoming call cannot be answered. When I pick up the handset to answer the call I hear nothing. The caller reports (after I call them back) that all they heard was continual ringing - the call never went to voice mail either. I can see their caller ID and that is how I can call them back.

I see no errors on the system, their have been no changes or upgrades to the PBX, the phones, the network, or the router with the exception being an user change. This system has been static (as far as changes go) for about 7 months now and worked perfectly up to about a week ago. I did rename a phone (on the phone) about 2 weeks ago and also renamed an extension to a different user at the same time. Can that possibly cause this?

In the last 7 days a total of 735 calls have been made through the system and this problem has happened about 7 times.

I am using FreePBX version I have 2 trunks from for all calls. For the most part it is a stock install. I do have the IVR module up and running as well as day/night and ring groups.

There are about 20 extensions on the system. All incoming calls ring into one extension and if it is not answered they go to voicemail after 15 seconds of ringing. They also all go to voicemail after hours.

I don’t know where to start troubleshooting this problem… advice?

This is the second post describing this problem without any replies. I have the same problem, but it is permanent. Strangely, if I set the route to go to the phonebook, then it works. Have you found a solution?