Is there a way to buy bitcoins using mobile phone credit?
The problem will likely be that mobile phone airtime/credit is usually reversible at some level. For instance, an M-PESA transaction might be reversed under certain situations such as claiming a phone was stolen and the funds sent without authorization.
So where you see this first done, it is usually by individuals selling who will find trades using something like LocalBitcoins or MyCelium app's local trader feature.
It will therefore be spotty for some time -- a trader might be found for one mobile airtime/credit network but not the next. Which mobile airtime/credit network were you looking to do this with?