Marlene has been through 3 iPhones and she is constantly having issues being able to hear her calls after 15 minutes. Leo says it's unlikely that it's a phone issue since it happens from phone to phone. Leo suspects that it's an issue with her wireless carrier (T-Mobile). The SIM card could also be the culprit. If she's replaced that, then it's still a mystery.
In the Chatroom, one member says he has the same issue with MetroPCS which uses the T-Mobile network. So it has to be T-Mobile. If they can't fix it, they may not cop to it.
One thing Marlene can try is a cell phone diagnostic code on the mobile phone. She should do a Google search for it. That may help her to diagnose the issue.