In one embodiment, a method includes receiving, at an IP private branch exchange (IP PBX), an event notification message from a user agent corresponding to a voicemail system. The event notification message includes a Request-URI field identifying a Uniform Resource Identifier (URI) of the IP PBX and a header field identifying a target mailbox. The method also includes identifying a URI corresponding to the target mailbox and forwarding the event notification message with a Request-URI field identifying the URI corresponding to the target mailbox.