Phone freezing after calls during lock screen

[tjnbama]

If anyone has any idea what the cause could be PLEASE let me know.. I have 2 HTC Design's purchased at the same time. One is perfect - love it! The other is a nightmare. Everytime a call comes in during the lock screen, the call can be answered but the instant you end the call, the entire phone freezes. If you try calling the phone, you get a message that it is unavailable. I have to pull the battery or plug in the cable to get it return to normal. I have done everything including exchanging it for a 2nd phone that does the exact same thing. Multiple factory resets, removed all apps, installed all updates, etc. Finally, I removed the SD card and contacts, factory reset and removed apps and with a completely bare phone, same thing. Sprint has updated the data profile and doesn't have a clue. Any ideas are greatly appreciated!