Android users can enable mobile phone billing in the Google Play App. Open the Google Play app > My Account > Add payment method > Enable 'your carrier' billing. Your carrier needs to be supported by Google Play (for example, Mobily, STC, and Zain are supported in Saudi Arabia). See the list of supported mobile operators (Direct carrier billing > Direct carrier billing availability). 

Apple iOS users can set up other payment methods supported by Apple, including Alipay, UnionPay, and mobile phone billing (through supported carriers).