The invention provides an apparatus for transmitting calls generated by a first communication network over a second communication network to a third communication network, the first communication network operating in accordance with a first communication protocol which enables call services, the third communication network operating in accordance with a communication protocol which enables at least one of the call services enabled by the first communication protocol and the second communication network operating according to a second communication protocol different to the first. The apparatus includes means to produce a data message compatible with the second communication protocol indicative of the required service and means to transmit the call and data message over the second communication network to the third communication network, where the data message is read by further apparatus and an appropriate call service established to the third communication network. By providing means to produce a data message compatible with the second protocol to carry the call service information it is possible to establish a call service over a communications network even though that network does not explicitly support the call service.