I get it, but it’s not always about cheap or about “pure” SIP (both of which I favor).
Slack is cheap. Microsoft Teams has already surpassed it. Those 13 million users of Teams have a tight integration with the rest of Office and their company directory and a good voice/video meeting system.
Guess what happens when they see that there’s a phone built in to Teams (which needs a PBX or PSTN service).
Sure, there is Zulu, or free desktop and mobile softphones, and they are cheaper. But they won’t integrate as nicely, especially if you are using Teams all day for your chat, meetings, and calls.
Ultimately the connector is just a SIP gateway that will work fine without requiring you to deal with a “butchered, half-baked protocol.” (saving the debate about the evolving nature of Internet protocols for another time)