I was also looking into to this in past and found a major issue which unfortunately isn't in anyone's power to resolve.

In my country there even has been a service like this, the fees were not ridiculous (10% markup if I remember right) but the worst thing is unbelievably high number of reversed transactions. It's very easy for anyone to buy bitcoin over SMS and then call his provider and cancel the transaction. There's unfortunately no way to protect against this since all those fiat transactions are managed by the cellular providers in-house and only then managed by some payment gateway service...

This is one of many things that need much more time and better general knowledge about crypto to be feasible.