I need the client to send SMS with specific information on the number and that's it. I using a PHP script to activate the service (normal recording MySQL).
Or you connect to a payment system which allows. And they take a percentage.
Or negotiate with all service providers (and everyone needs a contribution of a million dollars).
Or create your operator, connect to omeopatici network and put the horse fares.
Of all options - only the first is almost free (only 50-70 percent is not you).
All other - requires millions (in dollars) cost of setting up/connecting.
And that's just in addition to the technical costs of creating the corresponding SOFTWARE/support servers.
And you need to rent a shortcode. It is also not free.