A strategy to connect an asterisk and a propriarity pbx is setting the Asteriskbox in front of the propriarity pbx .
I assume that you have a isdn30/24 connection (europian/rest of the world).
You need a Dual E1 card (Sangoma, Digium) in the asterisk server so you can move the E1 of the provider to your asterisk box. The other E1 connection plays provider for the propriarity PBX and with an isdn cross cable you connect the asterisk server with the propriarity pbx
isnd connection (inbound/outbound) <-> (et mode) asterisk box (nt mode) <-> isdn crosscable <-> (et mode) propriarity pbx
You need some smart things in your dial plan especially when you have internal phones on your asterisk box and internal phones on your propriarity pbx that must be able to call each other but basically this is it.
This way you don’t have to spend money on extra ports on your propriarity pbx wich is most of the times very expensive.