Well bred there in the comments
In short, the topic here is looking at you a lot of money, so use the services of a ready-made service.
The cost of sending SMS from 0.6 to 0.25 for SMS depending on the amount of payment made.
API for sending is. You can also specify from whom this SMS.
You can order a short number for which you can receive SMS from users.
Only my advice to you, few people will take and send SMS to some number.
But if you send a text message to the user and tell him to enter the code on the site to unsubscribe from the SMS for additional information, then yes. But be sure to write in large letters on the website that if you enter a code from an SMS, you will not be charged 500 rubles as on many fake resources, such as winning an Iphone by sending an SMS to a number, and at the bottom in small letters about a subscription that costs 300r every month.
Why am I talking about the service? It is more profitable than buying a SIM card and equipment. Do you know the rates at which sending SMS per day more than 500 pieces will cost 25 kopecks per SMS? me not!