Send Text message from Salesforce through RingCentral API to the Cell Phone field

Do you mean to use RingCentral for Salesforce to send the message or through RingCentral API directly?

I am also interested in this. 
We have been looking at other app that provide this option, because it looks like you cannot do it with RingCentral. 
We want to be able to SMS clients their appointment and court information, as well as reminders, on an automated system.