Whilst you may be able to deposit via your phone bill or top-up credit, and as discussed, to only a limited amount, if you were to have a significant win or large balance you wanted to withdraw, then comes the next problem.
Making Withdrawals by Phone Bill on Casino is Not So Easy
That's not altogether a bad thing to keep your spending in check. The Pros of ease and convinience outweigh the slight con of low maximum deposits.