My interpretation was that the issue is the use of Microsoft Office's dominant position to (allegedly) push Slack out of the market. They aren't providing one p

My interpretation was that the issue is the use of Microsoft Office's dominant position to (allegedly) push Slack out of the market. They aren't providing one product that does two things (fax/phone), but rather they are providing multiple disparate products in a package together to take advantage of the fact that everyone wants one of them already. I think a better comparison is if Verizon were to decide they want to make a smart watch, so they start bundling them with every cell phone they sell. Sure, users can go out and buy a Fitbit if they want to, but Verizon would be taking advantage of their position as cell phone gatekeeper to get them into everyone's hands before they even have the chance to make a market decision. Users could come into a contract with a phone they got independently (analogous to windows users who don't use MSOffice), but the vast majority are just buying a phone from the Verizon store. Admittedly, the analogy would work better if Verizon was more dominant in the market, but as it stands the big three US cell providers seem to be on pretty equal footing.