I have finally managed to tie down Avaya and it apears the B179 SIP model, although allows you to configure an RTCP collector for monitoring, it's not actually supported. However, a newer release is due on the roadmap for a later 2.5 FP release which apparently will support RTCP.
It's also been confirmed that the B189 model(H.323) DOES support RTCP and should work with Prognosis.
Thanks for sharing out this very useful new info from Avaya about the B179 (RTCP support on Avaya roadmap for a later 2.5 FP release ) and the B189 model(H.323) (does RTCP already).
Just so everyone is on the same page here's the Online Help article for getting started with enabling RTCP from Avaya SIP Phones.
Prognosis for Unified Communications > Avaya Aura Communication Manager > Configuration > Avaya Setu...
Another thing to make sure of is that Prognosis is monitoring the Session Manager that the SIP phones are registered to. Prognosis should accept the received RTCP packets if it can associate them with the Session Manager.