Integrating asterisk with pbx ( Asterisk as gateway)

Our company is present in two countries. In Spain VOIP is already implemented using Cisco Call Manager. In Portugal, where I work, I need to install VOIP so when we call to the SPAIN Branch we will not have any cost. The problem is that we spent much money recently with our telephone system (I think is called PBX) and I just cannot substitute this system with asterisk and the normal phones with ip phones.
The scenario we pretend is this:

1- The employee makes a call for our Spain branch.
2- The PBX detects that the number is for Spain and send it to asterisk.
3- Asterisk then makes the call to Cisco call manager in Spain.

Normal phone ----> PBX -----> Asterisk —> dedicated line to spain—> Call Manager (Spain)

If a normal, non Voip call is made, the asterisk just don?t do nothing:

Normal phone —> PBX —> PSTN

Is this possible? What cards does the asterisk need to connect to our PBX? FXO or FXS?

The PBX we are using is A Siemens Hicom 150 E.

Anyone has implemented asterisk in this way? I would appreciate any help.