List of basic SIP commands -
INVITE : Invites a user to a call
ACK : Acknowledgement is used to facilitate reliable message exchange for INVITEs.
BYE : Terminates a connection between users
CANCEL : Terminates a request, or search, for a user. It is used if a client sends an INVITE and then changes its decision to call the recipient.
OPTIONS : Solicits information about a server's capabilities.
REGISTER : Registers a user's current location
INFO : Used for mid-session signaling