I have 2 sites with FreePBX Servers connected via an IAX2 trunk. Site A has extensions in the 1000-2999 range and Site B has extensions in the 3000-4999 range. Outbound routes are setup at each site so users can dial the 4 digit extension of either site and they are routed properly.
Each site has a receptionist ring group that rings several extensions. How can I dial the Site A receptionist ring group from Site B and vice-versa?
Make sure the context of the tie-line is from-internal at both ends and you have outbound routes to similarly cover those end points as you do for your extensions.
While my example only mentions the receptionist group, I need to do this with a number of different ring groups. The issue is that the ring groups for are in the 600-610 number range for both sites.
that was a mistake . it is best to have all your endpoints the same length and unique, juat kike phone numbers, now you have some custom coding to work a found the lack of uniqueness.
Yup. That’s the problem with not knowing what you don’t know 
As you can’t fix what you have, how about recreating your 600 series groups as both 9600 (remote) and 8600 (local) as appropriate and wait until your clients notice how well the new and normalized dialing patterns work