Calls are "stuck"

Hello everybody, this is my first post to this list. I've been reading it for a while now and I'm finally trying to get a bit deeper into GSM. We have a BS11 in the C4 (Chaos Computer Club Cologne) which I got fully working on Thursday this week. We had several mobile phones in the network and called each other and we could send and receive sms. All fine. Today I wanted to test another few things and everything got weird: I have 2 mobile phones authenticated in the network. One phone can call the other one, it rings and after accepting the call, the first still plays the "ringing sound", while everything on the second is silent. Vice versa, if I call the first mobile from the second, it dials endlessly. Sometimes OpenBSC segfaults. Before it does that, it tells me on the console that timer T303 expired. Another thing are those masses of "mISDN_send: error -12" messages in the kernel log. We got those on Thursday, too but they weren't important to us, because everything worked. Nevertheless, sending and receiving SMS still works. Also, the network sometimes is hard to find on the mobile phones. We blame a bad calibrated oscillator for that. As soon as we got an HFC-S card, we will try to recalibrate it. Is it possible that all those problems show up because of the oscillator? And why did it work really well on Thursday? So far, I tried a few obvious things like rebooting, both the computer and the BS11, cleaning and recompiling everything and I even deleted the old HLR database. I really don't know what to do else. <Insert helpless cry here> ;) Does anybody of you have an idea what I can try? We are running the latest OpenBSC from the git on a Debian testing with a stock 2.6.32-5 kernel on i686. Greetings, Andy