You could also use just one unit to do the trick, but remember that you get only half of the channels.
Since both calls will be VOIP, you could only get 4 calls from the A800.
There are some cheap gateways that you could use for the side where you do the “landing” of the call, and then use the Quintum for IVR and billing.
If you want to try on the A800, do the following:
Create on PBX group with four ports on it and a “landing” number
Crate a PSTN group with four ports and setup the IVR and billing
The first one will be getting the calls from I.P. to PBX, the second group will have IVR and billing.
With short telephone cords, loop from the PBX ports to the PSTN ports used on the experiment
The call will be coming VOIP, come out on the PBX ports and ring on the PSTN ports, trigering the IVR.
God help you with the disconnect issues and the quality of the calls.
Only slightly better than string and cans.
But some people use it anyway.