Thanks guys for all of your help.
Even Nortel TechSupport insists that STP should be enabled on MLT Trunks but Disabled on SMLT and IST.
Referring to those Configuration Guides, they create a lot of confusion some tell you to disable MLT's, where others recommend leaving enabled. I found they have a lot off mistakes, which makes things hard to understand. Especially for guys who just entered the world of VoIP like myself.
I left spanning tree disabled on my uplink ports 47,48
Thanks again