General :: Phone Freezes On Making Or Receiving Voice Calls

I have a Samsung Galaxy SII, on O2 in the UK with their latest software release (Jelly Bean). Since the update the Jelly Bean the other month, it had been working like a dream, but starting this week, my phone seems to freeze whenever a call is recieved, or I try to dial out. The screen just goes black and the phone becomes unresponsive. Sometimes, the screen just stays black and I have to take the battery out to restart. At other times the phone will come back after between 1 and 4 minutes, after apparently rebooting itself (the Samsung welcome screen flashes up).

The phone will also now hang and fail to play media (MP3) files. Whether using the Google Player, the native Samsung app or VLC. Normally, the screen goes black for a minute or two and then the app will show the track as queued up with artwork) but it will not play and the app just freezes. Other apps, internet browsing and text messages works as usual.

I have tried removing any recently installed apps (only Dropbox was new recently) and I tried the factory reset. The problems persist after both.