What was happening was that if a SIP station was busy, incoming calls from Sprint phones would get a busy indication and not use call forwarding (to go to voicemail or another line).
The SIP station was setup originally for forwarding using service type “SPR- voice (DSS1)”. When we added service type “TEL3K1 – telephony (3,1 khz, anate)” then the forwarding would work. Conversely, if we removed SPR and left TEL3K1, then only Sprint callers could get through. I killed a couple more brain cells and found that Sprint is sending a progress message in the ISDN SETUP message indicating the “Origination address is non-ISDN”, which was not in the SETUP message from an AT&T call. So I suspect this was the cause. Alternatively, you COULD just enable every forwarding type!