Phone is auto-muting when I am in a conversation.

In the last 4-6 months my phone has been muting while I am in a conversation. I have now changed out the phone twice in the last two months. I just purchased a new Nokia 6230b on Saturday with a new sim card from ATT as of yesterday also. Everything/all hardware has been changed out. Problem still exists. I do not have a solution. Any suggestions or solutions. This appears to be a auto-muting issue.