CICS 6.1
Polycom/Nortel conference phone model 2501-03308-001A, non-display model

I have a client with a Nortel conference that phone does not ring for a second incoming call. It ought to have two line capability. They are able to place a second outgoing call and they do have two "intercom" buttons set up for that extension, because with an M series on the same extension to test, they are able to hear a second call ring in. Can anyone tell me if this model does not allow a second incoming call, or what the issue may be?
Also, I have searched on line for a manual for this phone which could provide an answer, but to no avail.