By using mobile recharge api or prepaid recharge api service, your application can topup airtime/talktime on any Indian mobile number. It can recharge any Indian telecom operator with any valid denomination you like.
Benefits or how much you can earn in mobile recharge api business depends on your sales or volume you process via your website or application. More you do sales, more you will earn. Secondly, your income also depends on how much margin or commission you keep for yourself before distributing to your customers in B2B model or how much transaction charges are applied by your payment gateway provider in B2C model. B2B means you are doing business in your local area where you setup your own distributor network or retailer network or outlets. B2C means your business is completely online where you serve your customers directly by accepting payments from them online via payment gateway and processing their recharge request via api. So, if you are doing B2C, then make sure to keep your payment gateway transaction charges as low as possible, avoid buying payment gateway which have higher transaction charges or TDR. Even 0.1%, 0.2%, 0.3% matters alot in mobile recharge business. If you are doing B2B, you can distribute margin to your chain according to your needs or market demand. Offering higher margin to your chain and keeping yourself less will lead to more sales but less profit per transaction but if sales are good then ulimately you can make good profit at end of the day.