I see that currently no one sells Bitcoin using phone credit, and even if it does, I don't guarantee it will be very good to do so at this point, I think if you really want to buy Bitcoin, it's better to use an exchange like Binance, KuCoin , Bittrex, and other exchanges because existing exchanges like the ones I mentioned are real, if you buy on an exchange that actually has nothing to lose, but if you buy them on a site that doesn't seem real, you are at great risk of losing money.
But with phone we can access internet also penetrate in those platform, what are the reason you portray that know one sell bitcoin with phone credit?
We all know binance as exchange were we can buy bitcoin, even luno, you can buy bitcoin and sell bitcoin as many you want.
You can equally buy and sell any crptocurrency with bitcoin ATM machine if you have any around you,
because ATM transaction always move faster.