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
As user said my comment helped him and post it as answer.  

Try to install the rocket dial and in settings try to make it default app for receiving calls.
 Settings&gt;caller ID

Also you can use free version of HD Caller ID which has option to overtake the default caller screen.