R2 MfC signalling stands for Register(type2) Multiple frequency coding technique
MFC signaling is used for transfering of necessary information to establish a connection is performed on a general end-to-end basis by the compelled multifrequency coded register signals.
The first backward MFC signal has it own meaning and at the same time it serve as acknowledgement for the first forward MFC signal.
The frequencies of multifrequency (MFC) signals are the same as specified in the CCITT’s recommendation Q.441.
For more detail please refer to Following ITU recommedation site