The SIP-based multimedia communication system for providing mobility using lifelong numbers provides mobility through SIP network service domains and a global domain. The SIP network service domain comprises a user agent and an SIP network server. The user agent transmits request/response messages between users to set up, correct, and cancel calls, and when contact information of the user agent is registered and registration of the contact information is cancelled, requests to be informed of state changes in registration information. The SIP network server performs message routing between user agents and informs the contact information of the user agents during message routing. The global domain manages the SIP network service domains and allocates global SIP identifiers, that is, lifelong numbers to users to provide user-centered mobility.