The invention relates to an arrangement for mobile telephony and more exactly an arrangement to utilize mobile telephone systems in office telephony applications. The invention especially can make possible a combination of local mobility under a private branch exchange (PBX) and ordinary mobile telephony with the same terminal, i.e. mobile telephone. According to the invention the arrangement has a subsystem which includes a base station system (BSS) which is of mobile telephony type for communication with mobile terminals, a mobility server to handle the base station system and provide access to at least one public telecommunications system (PSTN) and one private branch exchange (PBX) to provide PBX-functions in the subsystem. The mobile terminals by that will become mobile locally under the private branch exchange.; Preferably the mobility server also provides access to at least one public mobile telephone system (PLMN), at which the mobile terminals are also mobile under the public mobile telephone system.