iPhone + Nextel PTT

Even if Apple added PTT features to the iPhone, PTT is not interoperable between carriers. AT&T doesn't, to the best of my knowledge, offer PTT at all. Verizon does so theoretically, that could work. It still wouldn't solve your problem, though, because it sounds as if you're currently communicating with Sprint customers and Sprint doesn't offer an iPhone.

You can do better than iPhone+Nextel. It is a FREE (yes free) app named TiKL Touch to Talk. Recommended by a couple of friends with Android on T-mobile and VZ, TiKL is the #1 communication app on Android for months. I was jealous when they were having fun with it. It is finally available on iPhone! And I am not disappointed.

As if free PTT on iPhone is not enough, TiKL lets you do all the following, all for free.
* group PTT
* international PTT with your facebook friends. I even TiKL'ed one of my facebook friends in Hong Kong (I am in New York).
* works on iPad and iPod Touch too

I tried all the PTT apps on iPhone. TiKL is hands down the best. Nothing else comes even close to the speed of TiKL. It works just like Nextel except that it works across carriers, for FREE! Doesn't get much better than that. Hope you will have as much fun as I am having with it.

There's an app for that:

http://itunes.apple.com/us/app/heytell/id352791835?mt=8