The easiest way to make that configuration work will be if both Quintums are on static I.P. addreeses.
If only one of them is on static I.P., the other should be on PUBLIC,dinamic I.P. address.
Or for a very complicated setup,one Quintum on static public I.P. and the other behind a NAT router.
The rest is easy.Setup a phone number on each Quintum.
Asociate the phone number to the actual FXS/FXO ports.
The create a static route pointing gateway “a” to the phone number on gateway “b” and viceversa.