The unlimited plans existed, but it was more expensive than the limited plan I used, since I didn't go over the limit.

There were big variations by carriers - that certainly wasn’t ubiquitous by the time the iPhone was released, because iMessage sidestepping it was important to enough people that everyone mentioned it in their reviews. For example:

> Besides ease of use, there's another side benefit to this seamless integration. If you send messages regularly to iOS 5 users, you may be able to switch to a cheaper texting plan from your carrier. Assuming you send messages exclusively to iOS 5 users, you may one day be able to ditch a texting plan altogether.

https://arstechnica.com/gadgets/2011/06/more-upcoming-ios-5-...