Call waiting and sip peer

I have Asterisk PBX A that my phone has the registration with (via SIP). I then have a SIP peer to Asterisk PBX B (using an extension off that PBX). When am using that SIP peer for a call, if someone calls the extension or DID tied to that extension on PBX B they get a busy signal. What I would like would be for the call to be forwarded over the SIP peer so I can deal with call waiting on the PBX A side (no inbound connection is seen by my PBX A from PBX B).

Thoughts? Not sure how best to proceed to troubleshoot / fix this. Thanks in advance!


so if you using the DID, and people call in on the DID, they get a busy tone. Surely that ‘busy tone’ is from the tel company, even before it hits PBX B? Or can you have multiples on the one DID.