Abstract: The invention relates to a prepayment indoor telephone in which payment is performed by means of memory cards. In order to make the use of the telephone set (104) secure, a verification module (110) is connected to its telephone line (130) in the vicinity of the exchange (102). The verification module (110) and the telephone (104) periodically generate two respective versions of authentication information and these two versions are compared in the verification module (110). Authentication takes place only if the card reader (128) has delivered a signal indicating that the required telephone charge has been paid.