How about to perform the call transfer operation (screened and unscreened) and trace the communication it will better on Nortel PBX side, in order to know which point is causing call transfer failure.
By the way these kind of features is complicated to handler on interconnected system, I just mean is no easy way for check intersystem resource state, as other than inband tones, mainly PBX system interconnect to others system by trunk resource and communication between system are handler by Trunk to Trunk PBX system control operation, PBX only use remote state (Busy, Congestion, etc.) for management own Trunk to Trunk resource operation.
Just in case of these kind of scenario it can be done through E1 (DR2 MFCR2 or DTMF) or ISDN PRI QSIG, lastone is more convenience because it cover more supplementary services than DR2 as; Completion of Calls to BusySubscriber (CCBS), Call Hold (HOLD)—by ISDN etc.
Mc GRATH Ricardo