"buy it or don't" does not work with oligopolies. If you had a free market, I would agree, but you very much don't in this case.
Why do you believe there is no free market on mobile phones? I mean, what exactly forces you to pick an iPhone over anything?
Don't want to sign up to one multinational behemoth? Well, your choice is to sign up to one other multinational behemoth.
I don't think that the manufacturer assertion is true at all. You can go to any random online store and get dozens of brands and manufacturers. It just so happens that popular demand focuses only on a hand full of manufacturers who are outcompeting everyone in the free market.
Take a look even to Android's market share. You have four manufacturers with double digit market shares, and a couple of dozen entriee. Is that what you call a monopoly?
https://www.appbrain.com/stats/top-manufacturers
The OS comment is even more badfling.
https://en.wikipedia.org/wiki/Comparison_of_mobile_operating...