That is fairly easy.
Run some single line extensions of the PBX to the PSTN ports of the A400.Ask your friendly PBX technician to put those extensions on a easy to remember number, and in a hunt group or rollover.
If you have a good PBX, you may ask the PBX tech to setup adn “access code” something like the number 32 to access those extensions.
On the Cisco, enter a new H323 gateway, and create a route such a way that when someone on the VOIP side dials a number that matches the range of numbers on the PBX, sends the calls to this gateway.
On the Quintum, on the PSTNTG group one, include all the cahnnels.
Set up IVRt type to second dial tone,
Create a hopp-off pattern that will catch the calls from the Cisco and send them to PSTN (really your PBX).
You will need to use huntpubldn and lamreplace fields.
On the other direction, when someone calls the extensions attached to the Quintum, they will get second dial tone, ready to call someone “on the other side of the fence”.
You will need to setup static routes on that direction.
Good luck!!!