This isn't radio, but it's very appropriate to our context. In South Africa we are partnering with our National AIDS Helpline to provide HIV counselling via MXit, a very popular mobile text-chat application used by millions of South Africans, especially youth.
We've noticed that sexual health questions come up a lot (should I have sex, genital warts, etc) and that youth are remarkably comfortable chatting with counsellors via text on their mobiles. It's completely anonymous, and youth can chat anywhere and no one need know. So you can be in a rural area, an informal township, on public transportation, but you can text-chat.
It's also very cheap for the user (a counselling conversation will cost them less than US$0.05).
To read more, see http://mobileactive.org/case-studies/red-chat