A system, apparatus, and method is directed towards managing a quick registration of a mobile device over a network. The quick registration is directed at enabling an end-user to conveniently register a mobile device to be useable for such communications sessions, as an instant messaging (IM) session, and the like. The invention employs a pre-determined extension to a short code that instructs a server to register the mobile device sending the message. The message includes an account identifier along with a password to the account. The message may be a single message line, thereby providing a quick and convenient mechanism for registering the mobile device. The server receives the message along with a mobile device identifier, and stores a mapping relationship between the account identifier and the mobile device identifier after validating the password. In one embodiment, the message includes a command that the mobile device also be logged into an IM session.