The method of receiving SMS payments?

Google SMS billing rubilling.com/tariffs
operators charge around half the fee for these fees
this method of payment has long been associated with fraud,
better look for other options

still have payments mobile phones for 5-6%, but there is approval needed from each hundred of the operator https://oplata.tinkoff.ru/landing/tariff

Google on the topic pseudo of subscriptions at mobile operators

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.

https://www.paysera.com/v2/ru-RU/sms-platezhi
and other similar operators.
micropayments SMS
payments SMS