There are a variety of ways to deposit money to play mobile slots:
Phone Bill: Deposit using your phone bill by charging to your airtime credit.
Credit / Debit Card: Register a card within mobile slot games and away you go.
Ukash: Also known as electronic cash, buy a voucher from a shop and enter into your slot game.
In addition, some mobile casinos may let you deposit using Click2Play and Neteller amongst others.
Read more about banking.