In the context of SIP,

RFC 3261, section 6

[a] client is any network element that sends SIP requests and receives SIP responses. Clients may or may not interact directly with a human user. User agent clients and proxies are clients.

Note that this view of the world differs somewhat from that of other protocols. In SIP client and server are merely roles that are played by devices for a specific interaction. Therefore, a user agent is both a client and a server, as is a proxy.

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