About your configuration to provide call forwarding from one location to another using VOIP:
1) Caller dials location number one, goes via normal phone line to the location, where is conected to the FXO port of a gateway. The gateway will convert the call to data, or VOIP and send the call back to the same phone line as ADSL data now, going to the second gateway on location two.
Similar setup on location two, the call comes in via ADSL, goes into the gateway and comes out via the FXS port of the gateway, to a phone or fax machine.
In order to do that, you will need a gateway that have FXO and FXS ports on the same box. Also, they should be able to work on a peer to peer configuration, without gatekeeper.
I read about many happy user of Qtelnet boxes doing that.
In the case of calls from office or location “a” to location “b”, the person will pick up the phone attached to the FXS port and call the other end.
In order to get the option to dial a digit and get re-routed, the only way that I could think of doing that is using a PBX or small voice mail system to answer the call (called pre-attendant on the US), and let the voice mail or auto attendant let ring a single line set or route you to the gateway to send the call to the other office.
Maybe someone could refine the original idea and come out with a better or cheaper solution.
And my normal disclaimer: Stay away from Quintum A200, unles you enjoy troubles.