Xperia Mini/pro :: Unable To Receive Calls After Long USB Tethering Period

Here are some details about the phone:
model number: SK17i
android version: 4.0.4, build number: 4.1.B.0.431
baseband version: 8x55A-AAABQOAZM-203028G-72

I use USB tethering a lot, and sometimes for very long times. When I disconnect the phone, sometimes, my phone becomes unable to receive calls. And even worse, the caller hears the normal calling sound, but my phone just doesn't do anything. So to the caller it seems that I'm just not answering, although the phone isn't notifying me in any way, during the call or after the call! So there is no way telling how many calls I'm missing..

When I try "reset" the mobile network by going to flight mode and back, the phone completely locks, and I have to remove the battery to reboot the phone. This problem won't go away unless I reboot the phone. This happens awfully often, regardless if I click USB tethering off from the phone first before disconnecting it. The most important function is not working and makes the "phone" useless.