Very Unusual Audio Issue

I have a very unusual audio issue that started happening about a week ago that I think has nothing to do with anything on my side, but I want to rule everything out.

I have one very specific case of intermittent audio failures (one-way/no audio) that only occurs when I have an inbound SIP DID route pointed at an extension on a Cisco SPA phone and it receives an inbound call. All Cisco SPA phones (I have 942 and 962 models) have this problem and I stress that it is very specifically only when receiving an inbound SIP DID call directly and only on about 70-80% of calls. Everything is fine if they call out, receive an internal call, or receive an external call if it goes through any type of other call flow before going to the phone (the transparent work-around I cooked up involves pushing the DID call through a blank announcement before going to the extension). Further, the Polycom phones I have work no problem and they cannot duplicate the same issue.

There were no changes on my side before this started happening and my SIP provider refuses to indicate if they changed anything on their side. The work-around is fine for now, but I’m a little OCD and utterly stumped as to what the heck is happening. The only thing I can think of on my side is to update asterisk (currently on 10.12.1) but due to an issue on the testing and development server I can’t do an update at this time.

Thanks to anyone who may have a suggestion of something for me to try!