It&#x27;s the open standard for VoIP.<p>It&#x27;s used in many places, but mostly offices. An office might have an exchange system, with features like voicemail and routing different types of call over different networks.<p>I have a personal account which gives cheap international calls, which I added to my android phone. I can receive calls at my SIP address, from anyone on any provider. When I make a call, I&#x27;m given the option of using the mobile network directly, or SIP.<p>Naturally, neither the phone networks not the big tech companies want you to use SIP. They&#x27;d rather you used normal calls, or their proprietary system.