Landline cannot place calls to AT&T cellphone
We have landline in our office and we are unable to place calls to any AT&T cellphone. When calling any AT&T cellphone, we receive a busy signal. We are able to call cellphones through other carriers. We have cell service with AT&T. I have confirmed that none of us have blocked our landline number. I have also tested and know for sure that I am not on another call and have full service at the time we placed the call.
We have been working with the provide of the landline and they are not able to resolve the issue. They suggested it could be an AT&T issue. Who can I contact about this strange situation?