Being an end user, I've never had to contact Avaya or Nortel support.
As I understand it, the support process for Avaya goes something like this:
Someone needing assistance with an Avaya product would first contact their Avaya Business Partner. If the BP can't resolve an issue, the BP contacts their distributor (either Catalyst or Jenne). If the distributor can't resolve the issue, they would open a ticket with Avaya.
I don't have any insight into Nortel's support policy.
Just like Avaya's equipment, there are tons of Nortel products in use all over the world. Nortel has had the Canadian market pretty much to themselves while Avaya has been a small player there.
Here in the US, I think Avaya is more popular of the 2 brands (Avaya is the modern incarnation of what was the business/commercial telephone division of Western Electric).
I'm not sure how would/will this affect Nortel products. As of late, Avaya has been inching away from traditional telephone systems in favor of hybrids.
Avaya retired the Merlin Magix in 2006 because it competed against the IP Office platform. Then Avaya released the Partner ACS R7 processor and we all know what happened there.
To Avaya's credit, the Partner ACS R8 processor seems to be bug-free. I haven't seen any reports of major problems posted here, or on any of the other tech help sites I participate in.
All I can say is "Stay Tuned".
If it ain't broke, I haven't fixed it yet.