Phone going into Emergency calls only after USB tethering for 30 minutes

Since the time I started using USB tethering, my phone has been going into Emergency calls only after 30 minutes of little to no use. I find it a bother to always have to restart my hone to use calls or USB tether again. Why is this happening? Is this a limitation of USB tethering? This is NOT a limitation imposed by my carrier. I am using a Sony Ericsson Xperia X10i, running stock upgraded Android 2.3.5. I appreciate any help.