Roaming support method and system in UMTS

The invention relates to a communication method and system, network element of roaming user is attached to the interviewed the first network, and possibly position of the service device service ancestral home network. When the network element of roaming user logging on interview network of called network element, while starting or called network unit connected with with such as automatically device and network element of interview network message transmission to a ancestral home network, and is called network element information, or the interview network service information unit. Are the information is connected with the is taken the protocol message transmission, especially a message transmission for session starting protocol.