The phone company is the sole-source provider and you couldn't opt for another platform. The more relevant analogy is soda fountains in a restaurant - the provider chose Coke, but I want Pepsi. (My OSS founder wanted Slack, I want discord.)
That's a different medium, although I understand your point: Slack is the sole source provider of communicating on the Slack network. Personally, I don't think this is a problem or should be fixed because of how I see the tradeoffs and side effects. Similarly, I want Apple to run their own app store and not allow sideloading because I prefer the set of tradeoffs that come with that, versus the other reality.
