Cant answer the phone

Often when someone phones me, the phone just locks up. I can't slide the ring up to answer. Sometimes the ring moves a little.. sometimes not at all. I just can't do anything until the caller stops ringing. The phone becomes responsive again and I have to call them back. I have just upgraded to 4.0 from 2.3 and the problem is still there. I haven't rooted the phone or done anything special to it.. Apart from this the phone is always responsive and I don't really have any other problems. How can I sort this out? Phone: HTC Sensation-xe