DTMF decoding on inbound calls stops working

We have set up a second trunk on our free pbx system. After adding the new outbound route the old trunk, which is still receiving inbound calls stops decoding the users dtmf codes. “that extension does not exist”. If I substitute the old trunk on the new outbound route the problem is fixed. I think that may be a red herring, but that is what cures the problem. I need some ideas on a more logical reason for the failure of the system to decode dtmf on the inbound call. Tnx. in advance.