We have had so many problems with registration/timers that we are doing the next one P to P.

I am being told NOT to set up a PROXY, but to set up a GATEWAY.

Any enlightenment on this?

I have seen both work for the same ITSP however if the ITSP itself is suggesting one works best with them, I don't see why you would decline to comply. The major SIP vendors should be able to advise you how to configure the Allworx to inter-operate with them.