Is there any cost for sending sms from sandbox

If you are using for application to person (A2P) you can use RingCentral Toll_Free sms feature

From Sandbox, the sms are for testing purpose and there will be a watermark on every sms text.

ref: https://developers.ringcentral.com/guide/basics/sandbox

You can refer here for your answer:

https://forums.developers.ringcentral.com/questions/9438/cost-per-sms.html

https://support.ringcentral.com/s/article/International-SMS?language=en_US

Thank you so much OP for the details. I also heard about it sending free SMS via website.