Abstract: A cellular radio telephone system comprises a mobile cellular telephone (1) adapted to have a hand portable telephone (2) containing a number assignment module, i.e. a NAM (7), removably connected thereto. The mobile telephone (1) has a microprocessor (4) which can access the information stored in the NAM (7) of the hand portable telephone (2) either directly or via a memory 16 whereby the mobile automatically assumes the telephone number of the hand portable telephone when the two telephones are connected together. Hence a subscriber need only have a single billing account for both telephones. Also, the same mobile telephone may be used as a host at different times by any number of subscribers having their own hand portable telephones. Each subscriber will be able to receive his own calls and the charges for using the system can be levied to the appropriate subscriber's account.