This is typical and expected behavior. It's also not surprising that the tech is sending you a replacement phone (although it is sad). EV (or EVDO) is capable of DATA only. When you receive a call and the call connects, your phone goes into 1x mode for the duration of the call. As soon as the call is completed, the phone 'should' go back into EV mode.

Now, if your phone is staying in 1X (an entirely different problem), a little more info in your original post might have been a bit more helpful.

Based on your thread title and your description, however, you aren't experiencing an error and don't need to be worried. As long as it goes back to EV after you hang up you don't need to send the phone back or call tech support.