I noticed for a while that a call will be transferred to my phone (Fanvil X3s) and it will ring for a fraction of a second and then show "missed call". A few seconds later I'll hear the switchboard ringing and the receptionist answering the call and putting it through to me. The first time the call cut on their side. First it happened about 1/20 calls. Now it's more 1/4 calls.
Now I'm getting complaints from clients (Yealink T-40/41) with the same issue. One in particular has a ring group as the destination for incoming calls and those phones also ring a fraction of a second and then drop (showing a missed call) and the person has to phone back in - not happy because their call dropped. This was also a rare occurrence and now it's happening more often. It's starting at other clients as well.