If only there were a struggling US mobile provider looking to be sold that already had a large Android user base...
In mobile OS, I think they can avoid being classified having 70% or anywhere near it since Android is technically open source and anyone can fork it, and several flavors such as the kindle version of Android aren't under their control.
In the carrier space, they'd be far from owning a lot of market share.
They just need to make sure that if they acquire such power that they restrain from anticompetitive practices afforded to them by their position.