How to do sms sending system?

Answer 1

you could send sms for free (for you sender: the receiver pays) with php but there are caveats such as you have to know the phone operator associated to the specific number you want to send the sms to.
https://code.tutsplus.com/tutorials/how-to-send-text-messages-with-php--net-17693

Answer 2

twilio.com is fairly cheap. Basically you rent a phone number, and send and receive messages using a simple REST API.

Answer 3

Thanks