Boku is a very safe payment provider. The company has been around for many years. It is regulated and audited by the Financial Conduct Authority of the United Kingdom.
All transactions are processed through secure connections. Each payment has to be confirmed by a text message which is sent to your phone. In some cases, it may also required to enter a pin.
Boku is also good for cost control, since the possible payment amounts are very low, and in case you're using a prepaid SIM, you can spend more than is on the card.