It is helpful to clarify the various terms used when discussing the SIP protocol:
MessagesParent topic: SIP Overview
Messages are the individual text exchanges that occur between a server and a client. There can be two basic types of messages: Requests and Responses.
A Transaction occurs between a client and a server and made up of all the messages from the first request sent by the client to the server up to and including the final response sent from the server to the client. If the request is an INVITE and the final response is a non-2xx, the transaction also includes an ACK to the response. The ACK for a 2xx response to an INVITE request is a separate transaction.
Dialog is a peer-to-peer SIP relationship between two user agents that persists for some time. A dialog is identified by a Call-ID, a local tag, and a remote tag.
The Call of a called party comprises of all the dialogs in which it is involved. It may be thought of as a session.
A caller may have connections to a number of called parties at a time forming a number of dialogs. All these dialogs make a single call.