A gateway with a FXO device on the place where the phone line is located, connected to a permanent broadband conection.
At the answering service end, you will have a gateway withh a FXS device,connected to the answering service console or phone system.
You go “peer to peer” from one gateway to the other.
You will need static I.P. addreess at least on the FXS side.
Will be better if in both locations.