If you are a SP, then it is free to provide downlink information to users through the gateway of China Mobile or China Unicom. Of course, the SP has the final say whether to charge or not.

If your mobile phone often receives commercial promotional information sent by SP short numbers (i.e. SP codes, not ordinary mobile phone numbers) (for example: reply to a message to a certain number, you can view "friend" photos—— This is purely illegal and inducing behavior! ), then you know that the SMS gateway is free.

SP, the abbreviation of Service Provider, can be literally translated into service provider.