A method of handoff across different networks is disclosed. A first communication link through a first network is used for communicating between a first user agent and a second user agent. The user agents negotiate to use a second communication link for the same call. The second communication link is established through a second network between the first user agent and the second user agent while maintaining the first communication link. Related data is sent through the first communication link and the second communication link. The first communication link is dropped and communication is continued using the second communication link.