Re: I see trouble ahead......
A two minute call at, say 80kbps, is 1.2MB. Double it for bi-directionality, but then (roughly) halve it for silence if the app supports it (which it should). That's with a fairly high-quality codec. Stick more aggressive coding in there and you can probably drop it down to 16kbps without that much issue. In that scenario you're talking about 300kBytes, which isn't all that much to trouble your data plan.
On Orange (UK), when roaming to France, a megabyte of data costs you 70p, so that two minute call on the more compressed codec will cost you 23p. If you made that call as a voice call it would cost you more than twice as much.
In Kenya, again with Orange, you may want to reconsider using this app. Here it is £8/MB vs £1.75/min for making a call.
In conclusion, if you know your tariffs in advance, you can probably make an overall saving as long as you appreciate that you'll be gouged for roaming either way :)