An interface circuit which adapts an existing telecommunications device, such as a facsimile machine, to successfully operate through an existing cellular mobile radiotelephone. No significant modifications are required of the telecommunications device or of the cellular mobile radiotelephone. The interface circuit includes circuitry to convert a two-wire communication interface of the telecommunications device into a four-wire communication interface required by the cellular mobile radiotelephone. This circuitry includes a received signal altering circuit which permits controlling side tone cancellation independently of transmitted and received signal gain control. In addition, the interface circuit includes a switchhook status detector which monitors the switchhook state of the telecommunications device to control muting in the cellular mobile radiotelephone. The muting of the cellular mobile radiotelephone prevents contamination of transmitted data from the telecommunications device and protects a user from hearing displeasing sounds associated with handshaking signals and data communication.