iPhone OS 1 was far from fully featured though from a phone perspective. It didn't have MMS.
I was talking about iOS being fully featured from the point of view of an operating system (I'm using the term in its computer science meaning -
https://en.wikipedia.org/wiki/Operating_system). From the start, iOS was a proper, mature, preemptive multitasking OS with proper memory management, etc. It is based on BSD UNIX, so can trace its heritage to the mini-computers of the early seventies. Other mobile OS's of the time, such as Symbian and Windows Mobile grew from the mobile platforms of the late nineties, and were technically limited by the capabilities of late nineties' mobile chips.
Sure, iOS1 didn't have MMS, but from the point of view of an operating system that's just an application and can easily be added.
Let me put it another way - iOS was different from earlier mobile platforms because its foundations were strong, being based on BSD UNIX. The same for Android, which builds on Linux. In comparison, earlier mobile OS's were built on metaphorical sand.