In this scenario, this can possible be an intermittent problem, that when a call generated from an IP phone to Nortel phone does not give any tone such as a Ring Back or Engage. The IP Phone receives just silence for some time and then a fast busy.
This can possible be a visio for this kind of network:
IP Phone--> CallManager----> E1 R2 Gateway(3800 Series)--> Nortel PBX--> Phone
It has been observed that when Line signaling is defined as R2-Digital or R2-Pulse, the requirement for R2-Analog results in the same behavior. In order to resolve this issue, change the Line signaling to R2-Analog.
Refer to these documents for more information about E1 R2 Signaling: