[xg15]

That's not how the network effect works. In that case, who is restricted isn't iPhone users but Android users who want to talk to iPhone users. Because they can't choose whether or not their contacts use an iPhone and communicating between Android and iPhones makes for a degraded experience (that the whole controversy is about), so in effect, Android users will be under pressure to switch to iPhones without them being able to do anything about that.

[woodruffw]

I talk with my friends with Androids just fine over Signal, WhatsApp, and several other apps. They haven’t expressed any pressure to switch.

But again: a network effect is not evidence of monopolistic or anticompetitive behavior. Twitter has (had?) a network effect, and could not meaningfully be said to have a monopoly over social media.