Thanks AdananI am currently stuck, but seeing your explanation is very helpful, maybe the 78xx type is not designed for other PBXs such as SIP propitary.

I also have the same problem, but I plan to register with Avaya Session manager.I am currently testing the CP-7821-K9= V04 type.Previously I had converted to MPP but it didn't work,maybe for type 78xx can not register to another PBX