by checking the validity of a code

Full Title

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) > with means for preventing unauthorised or fraudulent calling (verifying user identity or authority in secret or secure digital communications H04L9/32) > Preventing unauthorised calls to a telephone set > by checking the validity of a code