Don't think it's possible. The apps available require some sort of network, be it 3G, wifi etc. Not sure if you could 'phone in' to a base station to transmit or create an ad hoc wifi network (range would be rubbish even with yagi antennas), either way the other users would need the same app.
Even if the right frequencies could be found, the power output of phones is way too small for anything sensible. Think how easy it is to lose reception from a cell tower when there is only one on offer, and that it with a nice big receiving station. Phone to phone would just not work with the current powers.
That said, it would be possible to produce an off-grid network of mobile or fixed cells. But pricey.
Sorry for the digging up this necropost but thought I'd chip in my own personal knowledge on the matter,
Yes this is possible and it can be incredibly cheaply, assuming you've already got a phone, spare PC and a radio.
Just as Oscar said, Zello is a good solution.
There's quite a few people who have implemented Zello to radio systems and they work rather well. 2 summers ago I spoke to an English retiree in the South of France from by kitchen using the same sort of system. It was patched through PMR446 (the Europe-wide common licence-free walkie talkie band) and worked amazingly well.
Check out these two videos. The first one is more of a demonstration:
This is absolutely possible but not simple. I'd need to know more about your set up and how/where you want it to operate. Feel free to give me a call on 07768 501775 or email me email@example.com and I can answer all of your questions.