From as early on as iOS 4 up until the current iOS 5.0.1, this problem has found its way into all versions of the iPhone. After ending a conversation with a friend, you may find yourself unable to hang up or your phone may even freeze at the conclusion of a call. To fix these problems, you will have to follow these steps below.
Step 1) Hard reset your phone by holding down the home and the sleep/wake button at the same time until you see the Apple logo appear.
Step 2) If that didn't work, you will have to restore your iPhone using this guide here. Be sure to set up your iPhone as a new phone, if you set it up from a backup, you will inevitably have the same problems.