My client wants to send a SMS to his customers from his computerised billing system.
But, I do not know anythig about this subject, How it can do.
I want to know, how can I do that? And Which services I have to need. How do I correspond with these services by codification?
Please! help me.

Use a web service. Your app makes a request by using the service's API (essentially you just send the required data to a script on their server), and then the web service takes care of delivering the SMS. You can find a lot of subscription services on Google, a quick search turned this up: Twilo SMS

Check out Ozeki and download it, the best thing is that you can use it for free for 30 days then if you feel good with it you can than pay for it http://www.ozekisms.com/sms-gateway/index.php?owpn=585

Or check out create your own app then create your own voip system using sipwise then create account for your client system and then hardcode it to your app then point the app to send what to where. Here is a guide of creating your own voip system https://www.sipwise.com/news/technical/byov-services-1/

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