by generating or selecting currents of predetermined frequencies or combinations of frequencies

Substation equipment, e.g. for use by subscribers > Devices for calling a subscriber > Devices which can set up and transmit only one digit at a time > by generating or selecting currents of predetermined frequencies or combinations of frequencies

Substation equipment, e.g. for use by subscribers (subscriber services or facilities provided at exchanges H04M3/00; prepayment telephone coin boxes H04M17/00; current supply arrangements H04M19/08) > Devices for calling a subscriber (H04M1/66 takes precedence) > Devices which can set up and transmit only one digit at a time > by generating or selecting currents of predetermined frequencies or combinations of frequencies