You cannot create a sms service in asp.net alone. The SMS content has to be transmitted through the internet to the telcom operator and from the telecom operator to the other telecom network(s) which then send it to the receiving devices like mobiles.
Since its not feasible to setup a telecom network for the sake of SMS service alone, you need to buy the SMS service from the telecom operators. There are a lot of SMS service options available with flexible rentals/charges. You need to decide the geographical areas where you want to send the SMS and opt for a SMS service according to your budget.
Bulk SMS service[
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