Is there a cost per SMS sent?

You can check the details with the backend developer team by submitting a help ticket.

Meanwhile, you can refer this https://support.ringcentral.com/s/article/International-SMS?language=en_US

Please check the below link and also check with the developer team by submitting a help ticket.

RingCentral regular phone numbers are not meant to be used for A2P SMS messaging. Violating regulation may cause your number to be blocked by carriers.

Please use RingCentral Toll_Free SMS number as discussed in this blog.