Abstract: An interface unit for coupling an international-type pay-telephone to a land-based or cellular telephone system. The interfacing unit of the invention has a microcomputer, and an associated EPROM in which is stored the exchanges of the telephone numbers that are accessible by that local payphone-terminal, which EPROM also stores a table indicative of the tariffing rates associated with the respective exchanges called. The invention also includes the tone-detectors for detecting the dialed DTMF digits of the pay-telephone, for the conversion thereof for storage in the microcomputer for the determination of a valid telephone number dialed in order to determine the associated tariffing rates. The tone detectors also detect the tone-signals received from the telephone system. The invention also includes "isolating" switching circuitry for isolating the coin-operated pay terminal from its couplement to the telephone system until the telephone call has been connected.