Incoming Calls not forwarding to all lines set up to answer

We recently added an answering service to our list of phone #'s to forward calls to. When we test call using our office # it doesn't seem to be forwarding the call to the answering service # that was added. It rings on the other lines. When we call the answering service # directly (not by calling the company #) we get thru to the answering service. How do I fix this so that our answering service can actually answer the calls when they come in?