phone line on my account tries to call me and they get the message that they are not allowed

One of the phone lines on my account will attempt to call me and they receive a message saying that they are not allowed to call this subscriber.  Then they try again, and the second time alot of times I will get the call, answer it and speak, and she cannot hear me.  So I have to hang up and call her back.  Once we have actually connected then she will have no problem calling me for that day, sometimes two.  But invariably it will happen again, a new day she will call me and get the message, and have to hang up and call again.  My bill is auto paid, and this does not seem to happen with any other line on my account.