Cellular radio communication system comprising an IP Multimedia subsystem and a Multimedia Telephony application server communicatively coupled to the IP Multimedia subsystem, wherein - the IP Multimedia subsystem is adapted for receiving a REGISTER message (140.1) from a radio terminal, said REGISTER message (140.1) comprising an option tag indicating a network based service supported by the radio terminal and said option tag requesting information whether said network based service is supported by the cellular communication system; and - the IP Multimedia subsystem is adapted to acknowledge said REGISTER message (140.1) by transmitting an positive response message (140.2) to the radio terminal, and - the IP Multimedia subsystem is adapted to transmit a third party REGISTER message (140.3) to the Multimedia Telephony application server, said third party register message (140.3) comprising the full REGISTER request (140.1) and comprising the positive acknowledge message (140.2); and - the Multimedia Telephony application server is adapted to transmit a positive response message (140.4) to the IP Multimedia subsystem indicating receipt of the third party REGISTER message; and - the Multimedia Telephony application server is adapted to transmit a message request (140.5) to the IP Multimedia subsystem, said message request indicating support of the network based service by the Multimedia Telephony application server; and - the IP Multimedia subsystem is adapted to forward the message request (140.5) to the radio terminal; and - the radio terminal is adapted to update a local variable corresponding to the indicated support of the network based service upon receipt of the message request (140.5) and to transmit a positive acknowledge message (140.8) to the IP Multimedia subsystem; and - the IP Multimedia subsystem is adapted to forward the positive acknowledge message (140.9) to the Multimedia telephony application server; and - the Multimedia Telephony application server is further adapted to update a user profile associated with the radio terminal corresponding to the positive acknowledge message of the radio terminal forwarded by the IP Multimedia subsystem by storing, in said user profile, that the radio terminal supports the network-based service. - the IP Multimedia subsystem is adapted for receiving a REGISTER message (140.1) from a radio terminal, said REGISTER message (140.1) comprising an option tag indicating a network based service supported by the radio terminal and said option tag requesting information whether said network based service is supported by the cellular communication system; and - the IP Multimedia subsystem is adapted to acknowledge said REGISTER message (140.1) by transmitting an positive response message (140.2) to the radio terminal, and - the IP Multimedia subsystem is adapted to transmit a third party REGISTER message (140.3) to the Multimedia Telephony application server, said third party register message (140.3) comprising the full REGISTER request (140.1) and comprising the positive acknowledge message (140.2); and - the Multimedia Telephony application server is adapted to transmit a positive response message (140.4) to the IP Multimedia subsystem indicating receipt of the third party REGISTER message; and - the Multimedia Telephony application server is adapted to transmit a message request (140.5) to the IP Multimedia subsystem, said message request indicating support of the network based service by the Multimedia Telephony application server; and - the IP Multimedia subsystem is adapted to forward the message request (140.5) to the radio terminal; and - the radio terminal is adapted to update a local variable corresponding to the indicated support of the network based service upon receipt of the message request (140.5) and to transmit a positive acknowledge message (140.8) to the IP Multimedia subsystem; and - the IP Multimedia subsystem is adapted to forward the positive acknowledge message (140.9) to the Multimedia telephony application server; and - the Multimedia Telephony application server is further adapted to update a user profile associated with the radio terminal corresponding to the positive acknowledge message of the radio terminal forwarded by the IP Multimedia subsystem by storing, in said user profile, that the radio terminal supports the network-based service.