What is a SIP Proxy

A SIP Proxy is a special node or piece of software that manages SIP communications between the user device or user agent and the media server or PBX.

There is a number of reasons to use a SIP Proxy including
- improved server side handling of network address translation (NAT) between user agents and the PBX (removing the need for STUN settings)
- updating or managing SIP messaging with additional parameters or corrections where appropriate
- improved security handling through rejection of unwanted traffic before even attempting to authenticate
- good model for mobile devices with variable IP address allocations with SIP proxy providing an "Edge" gateway model of Session Border Controller (SBC) for the PBX
- Edge gateway model also allows protection of PBX by layering it back in the architecture with the SIP Proxy being the only entry point from the internet to the PBX
- load balancing of traffic to media servers through a SIP Proxy to multiple media server / PBX model

More recently, in the case of Microsoft Teams, the SIP Proxy can operate as a Session Border Controller (SBC) for your Microsoft Teams clients towards your PBX

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