Hi Chud,
I have the same issue as you, but I'm on Wind Mobile. This issue happens intermittently anywhere between 1 to 2 days, my phone will stop being able to receive calls. It's very frustrating and Samsung is really no help. After two attempts on trying to "repair" my phone, they have failed each time. The only thing they did was they ran a "RF Test" and everything passed, they said. So they re-flash my phone with the same ROM - again and again.. I told them, "This is my second time sending it in, and you guys are redo'ing the same steps as the first repair. You don't need to be a rocket scientist to know this, but wouldn't you think when you send me back the phone - the issue is still going to be there?!" ... And sure enough, when I got the phone back, I was still unable to receive calls..
So I took matters into my own hands - I flashed it with a older release stock rom and I tested it for 2 weeks, and it was able to receive calls and texts. When I flashed with the earlier release, it also flashed the baseband. Either way, I should not be prevented from using the up to date software! Even when you update to the latest version and flash the baseband manually with the older release (from H2 to L6), the problem still exists. This is a Samsung issue but if you ever tried dealing with them over the phone, it's painful. They said they would send me a REFURB replacement - (believe me, I tried fighting for a brand new phone, since I paid for a brand new phone and this issue came straight out of the box - it's not even my fault to begin with!). It's really a huge gamble when you buy from Samsung. You're just hoping you pick a phone that is not defective or you will end up with a REFURB for your replacement. Totally unfair. It's been 3 weeks and I still don't see any phone replacement nor did I receive any mails from Samsung.
Btw, Galaxy S3 is the phone I have.
If you discover a permanent fix for this issue, please let me know. I've been searching but no luck.
Thanks!