Why does my landline say, "the device you are using is not registered to place calls on the network?"

I  have AT&T for my internet connect and my landline goes through the internet.  Far too often, my calls are interrupted and I have to call the person back because we can't hear each other.  Also, I am now unable to place calls due to a message that says, "the device you are using is not registered to place calls on the network?"