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Companies use various ports for various reasons. For a VoIP setup, Port 9875 is registered with the Internet Assigned Numbers Authority for Session Announcement Protocol. Session announcements are used to define the format and describe the information that will be exchanged on a multicast conferencing session. The VoIP system traffic directed to this port will communicate the start and stop of a session if this is the port your system expects. Port 9875 uses the IP network standard Transmission Control Protocol, which guarantees the delivery of data packets in the order in which they were sent.
With that said, some ports are a bit more notorious for being exploited than others. Port 9875 in particular has the ominous distinction of being known as the Portal of Doom in some circles because the Portal of Doom Trojan can use this port to communicate and give an attacker control of a system. This port is also used to create and break down IP sessions. VMware uses Port 9875 to dynamically create sessions for the vSphere Web Client Java Management Extension and is acquired upon starting the vSphere Web Client service.
It is always important to determine what ports are required for various applications before you just block them off for security reasons. You can search the port number online and find the IP ports. Just make sure you read a few answers to determine which is active on your network before taking action in your firewall.
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