MFC / R2 is a Register to Register signalling protocol.
This was used with earlier Analog switches to send signalling
over a CU (copper) pair circuit. The earlier decadic pulsing is now replaced bydual tones (MF). MFC / R2 can be used with E1.0 circuits,however, a MUX is required to groom the R2 signals for the E1.0 Time Slots. For your case with PBX you either use Earth or Loop Calling to seize a circuit. For use with a PBX R2 can be used. You have to configure the circuit for CAS.
As the industry has moved on you will see that PBX/ISPBX/etc.. supports DASS2 / DPNSS1 / Q.Sig, etc..
Centrex was also a solution where PABX functionality was provided from the Switch.