The PBX ports of the A400 are identical to a normal phone line provided by the phone company.
They are two wires, loop start trunks.
You could connect the PBX ports f the A400 directily to any PBX to the normal ports where normally you will hook -up the Bell or phone company lines.
They are labeled C.O. trunks or circuits, depending where you live and who is your provider.
You could mix them togheter. That way the user of the phone of the PBX have a choice of using normal phone lines ,or dilaing a diferent access code, or selecting a diferent line, they could go the normal way or VOIP.
Or you could send all your calls to the Quintum, and decide withing the Quintum wich calls go via PSTN and wich ones go via VOIP.
A little bit more complicated, but that is what the Quintum is designed for anyway.
Best regards!!!!!