You will have to set the SAIL box up, effectively, as a carrier for the epygi box.
We have done this several times before with Multitech, Patton and a few others. Create a trunk using the Multitech template from carriers. This will build a peer only trunk that will allow the epygi to register with SAIL. Make sure you specify an IP address of dynamic in the peer entry.
On the epygi, create a trunk with a user-id and password which matches what you've created on the SAIL box and make sure that it registers with SAIL (otherwise SAIL won't be able to find it).
Route calls coming in on the Epygi FXO trunk directly out over SIP to the SAIL trunk.
Create a route on the SAIL box to send calls out over the epygi sip trunk.
We can't guarantee this will work but it has worked for all of the other Gateways we've tried.
Kind REgards
S