* & proprietary pabx

we would like to link a * server thru T2 to an existing proprietary pabx (MATRA NEXSPAN).

I would like to know how the * box will be able to communicate with the pabx, which would only act in a “slave-mode”. What language to use? Can we keep the existing settings so that we can simply plug the pabx back if * fails? Can we transfer the existing settings to *?