iPhone won't hang up?

I ve got a Verizon iPhone 4 running 4.2.10. When I make calls, the phone often won't respond after after a call. I will press end, and the button highlights - but does not do anything. If the other party hangs up, then the icons in the home screen wont respond. If I get a voice mail, the only way to hang up is to do a hard restart of the phone.

I tried resetting settings, and a restore with no success. I took the phone to the apple store. The guy told me that they've seen the issue before and that it is a software issue. But, he went ahead and replaced my phone. He suggested I try to restore from backup, and if that did not work to set it up as a new phone.

Well, the issue continued after i did a restore from backup. So, I set it up as a new phone and the issue continued. I tried to set it up as a new phone again but only put 5-6 apps on the phone to try to rule out an unstable app causing the issue.

I'm out of ideas. Anyone have any suggestions?