Not just a mobile phone, but a phone with either iOS or Android. Plus, a phone with an account with either of two parent companies (avoiding that on Android is possible, but far from trivial). Any "alternative" system is basically a non-starter as all these apps are typically only developed for these platforms, and it's become very hard for any new player to enter the market.
While the duopoly is better than the monopoly of Microsoft Windows back in the day, it's also worse as the reliance on software is much greater, and its become harder (if not impossible) to write your own implementation for $alternative_system.
