Boku deposit casinos are unable to provide withdrawal services to customers who win due to how their funds were deposited in the first place. Unlike with other services, such as e-wallets like PayPal or debit/credit cards, you aren't technically transferring money to the online Boku casino but rather adding a bill to your mobile contract.
As such, there is no way for Boku to reverse this process and 'take the money' off your phone bill. However, Boku online casinos all pay other methods for winnings to be withdrawn, such as bank transfers or e-wallet services.