I got this exact bug on many GRP phones with some UCM6300 on many deployement…
The problem we had is with the “Match Incoming Caller ID” setting that we configured to have internal call be on a distinctive ringtone. After some time they just stop ringing… We removed the phones from GDMS and factory reset, then configured them by hand and it worked for some phones. We had the same problem with Zero Config.
From what I understand it would be the Alert-Info Header that causes the problem right?