I have a similar issue for about 2 months on the SGS2. This is how it goes:
I first had CyanogenMod 10.1 with no problems at all until i installed total recall for the s2 (for legal purposes). Bad luck with that because after i installed it the phone didn't have any sound at all (music, calls, microphone, headset). I didn't have a backup so i had to reflash CM 10.1. Ok, for a while until i got reports that i don't receive phone calls, i was told that my phone was closed (even though it wasn't). Luckily i have a second phone. So, i restarted the phone, and it got back to normal. Next day, similar issue, and so on for a week until i decided to change the baseband. Did that, worked like a charm for a week and then it got back to not receiving phone calls. Decided to change to AOKP (06.15 - i later updated to 06.30 but had to downgrade to 06.15 again because android keyboard kept giving errors). No problems for a week, but finally the problem came back and persisted. Changed about 3 or 4 basebands with no luck. I sometimes check if i can send or receive calls with my other phone, and if i don't, i restart the phone and comes back to normal for a few hours or a day or two. Signal bars are full even when it doesn't work.
I got in huge trouble with this problem and i would really like a suggestion. Please help...