The XML scripts should have absolutely no effect on the FreePBX/Asterisk programming, They just make an elegant interface between Asterisk's state and the state of the phones through an XML server.
If *80 does not page anymore, then you must have changed something else.
By default from the feature code page, *54 and *55 will enable and disable the intercom feature of the phone (if supported), for paging to work intercom must be enabled on the extensions paged. Page groups you have setup can just be to be dialed from any phone allowed, all phones in that group will in effect be intercom'ed , does that work? check the various options on that page also.
All the above being equal, then again please read very carefully the Aastra XML manuals.
(I have no idea how Grandstream's work, my previous experience is that in general they do so only marginally and that only briefly. I tend to microwave them on sight :) , I just assure you that the Aastra script will not have anything to do with their non-operation, if in doubt, just read it, it does state it is only supported on 32 bit OS's at my last read, , I also don't use Elastix 2.0 yet in any useful fashion, so maybe you better await input from someone who does )
There are other solutions!!