The object of the call masking system for mobile telephones is to prevent the operation of mobile telephones which are present in a close environment without interfering with the normal operation of other telephones which are outside the closed environment. To this effect, the system generates interference signals which will prevent the mobile telephones from receiving the signalling channels sent by the base stations (for example GSM call signalling channel). The system monitors the power with which the carriers of the neighbouring base stations are received inside the room and will adjust the interference power radiating in the signalling channels so that the carrier/interference signal ratio will make the operation of the mobile telephones impossible. The system can be applied to closed environments wherein mobile telephones should not ring when receiving a call.