On a Cisco Unity integrated with Nortel PBX, during an attempt to record the CallHandler greetings on the public switched telephone network (PSTN) phone, the "The Specified phone number is busy" error message is heard when you click on Play of the media master. This happens when Cisco Unity ports are blocked on the private branch exchange (PBX).
This can happen due to the Cisco Unity port numbers that are blocked on the PBX.
In order to fix this issue, open the restricted Cisco Unity port numbers.
For example, change the NCOS value from 0 to 1.
Refer to these documents for more information on Cisco Unity and Nortel Integartion: