R2 signaling is a channel associated signaling (CAS) system developed in the 1960s that is still in use today in Europe, Latin America, Australia, and Asia. R2 signaling exists in several country versions or variants in an international version called Consultative Committee for International Telegraph and Telephone (CCITT-R2). The R2 signaling specifications are contained in International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Recommendations Q.400 through Q.490.
The two elements to R2 signaling are line signaling (supervisory signals) and inter-register signaling (call setup control signals). Most country variations in R2 signaling are with the inter-register signaling configuration.
MFC/R2 (Multi Frequency Compelled R2)
MFC/R2 is a peer-to-peer signaling protocol. This means there are just 2 parties involved in an R2, E1 link and both parties behave in the same way (unlike PRI where you have the "NET" and "CPE" sides).
The protocol defines 2 types of signals: Line Signal and Address Signal.
Line Signals are used to monitor the state of the call which includes the states:
Idle, Block, Seize, Seize Ack, Clear Back, Forced Release, Clear Forward, Answer
This is not a problem since state changes can occur only in a specific sequential order.
Address (inter-register) Signals consist of 15 different MF signals, which are audible tones composed of 2 frequencies that travel using the audio channel itself. The ITU defines which frequencies can be mixed to compose the MF tones and how to assign meanings to these tones. However, some countries assign different meanings to this tones. The MF tones are used to transmit ANI (Automatic Number Identification, commonly known by some users as Caller ID), DNIS (Dialed Number Identification Service which is the dialed number or destination number) and the Calling Party Category. As soon as the call is either accepted or rejected, the MF detector is turned off and no other MF signals will be exchanged, the audio channel can then be used for voice or early media.
Below is sample of the MFC/R2 call flow.