Very difficult to know why this fails without a trace. This software is fifteen years old and SIP implementation for subscribers was done differently (S0PP) than it is now (UFIP). Useful info could be
- What software is the 4K
- What loadware is the STMI
- What does the configuration look like for zand, feasu, sbcsu, cossu, sdat, actda for all subscribers
- What does the Wireshark trace show for the failure, is the transfer refused by STMI
- What does a 4K trace show, is the transfer refused by 4K
Without this information, it's a case of giving the old crystal ball a polish.