Presence systems, presence servers and presence agents

A presence system in a communication system comprising:

• a presence server comprising a directory of users on the communication system, the directory of users comprising a plurality of user identities, each user identity identifying a user in association with at least one terminal address, and each user identity being associated with a presence status that is either busy or available , wherein the users of the communication system may update information in relation to their identity and associated terminals through display endpoints;
• a first presence agent associated with a first Private Branching eXchange (PBX), the first presence agent comprising a first database comprising a plurality of user identities, each user identity identifying a user in association with at least one terminal address of a terminal connected to the first PBX;
• and
• a second presence agent associated with a second PBX, the second presence agent comprising a second database comprising a plurality of user identities, each user identity identifying a user in association with at least one terminal address of a terminal connected to the second PBX,
• wherein the presence server, the first and the second presence agents are interconnected via a messaging and signaling system and protocol, and
• wherein the presence server is configured to, upon request from a first user using a first terminal which is not connected to the first PBX, update information in the directory in relation to the identity of the first user and the associated first terminal,
• wherein the second presence agent is configured to, upon the second PBX receiving from an operator a call and routing the call to a second terminal, wherein the call was placed by the first user using the first terminal and to a second user using the second terminal connected to the second PBX, and wherein the call comprises a number of the first terminal and a number of the second terminal;
• retrieve in the second database the identity of the second user;
• and
• send to the presence server a request for identity of the first user, the request comprising the number of the first terminal and the identity of the second user,
• wherein the presence server is further configured to, upon request from the second presence agent comprising the number of the first terminal and the identity of the second user:
• retrieve in the directory the identity of the first user;
• change in the directory a presence status for the second user from available to busy;
• and
• return to the second presence agent the identity of the first user,
• wherein the second presence agent is further configured to:
• receive from the presence server the identity of the first user;
• and
• send to the first presence agent information that the first user should have a presence status of busy,
• wherein the first presence agent is configured to:
• send to the presence server a request for changing the presence status for the first user from available to busy,
• and wherein the presence server is yet further configured to, upon request from the first presence agent:
• change in the directory a presence status for the first user from available to busy.