The easy way to setup something like that will be using a couple of extensions out of each switch (single line phone ports, that provide dial tone from the switch), and a couple of trunk ports on each location.
The extensions out of the switch connect to the FXO ports of a external gateway, and the trunks connect to the FXS ports of the gateways.
That way, when someone in location “a” wants to call location “b”. picks up the phone and dial the access code of those trunks. Your phone man could set up a single digit accces code to do that. You pick up the phone dial something like “5” and get a second dial tone from the gateway. You dial the remote extension and your call goes VOIP, then trought the FXO ports, to the remote PBX and to a remote extension.
If you have caller I.D. or display phones, you could name those extensions “VOIP” or “remote office”, that way people will realice that they are getting a call from trhe remote site.
You could use a couple of Quintums for that, but i will not recomend the old A200. They are a piece of bad ingeniering on my personal opinion.
Quintum is getting a replacement for the A200. I do not know if they are available yet, but that could be a good solution for around $700 to $800 dollars for both ends.