You may be able to use your phone account to pay for virtual currency.

In your iTunes or Google Pay account, depending on your carrier and country, there may be an option to link your phone account with your iTunes / Google Play account, instead of inputting a credit card.

If this option is available simply follow the instructions on your iTunes / Google Play account set up. Then, when you make in app purchases, these payments will be billed to your cell phone carrier account.