<< need a different outbound proxy to the inbound proxy for each FXS port.>>
it sounds like you are wanting the fxs lines attached to the pbx to dial handsets on a remote spa8800.
are the 4fxs lines really going to a different provider for each line? like line one is going to proxy at ringcentral.com, line 2 goes to a proxy for bellcanada.net, and line3 to anotherprovider.com???
does your pbx have fxo or fxs ports on it?
a topology or drawing of what you have and what you want to do might be helpful.
It may be this is the type of configuration you are looking for, using IP dialing for a remote handset(extension) through spa ATAs.
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