In terms of opt-in, you will still be able to message any number just as you always have been, however the carriers require that you have permission from the recipient before messaging them.
Because the type of permission varies with the content of the message (for example, permission or consent is implied if they message you and you respond back to answer their question, whereas you need written and logged consent to send promotional or marketing messages) it is up to you to determine the correct level of consent required and obtain that consent prior to messaging the number. RingCentral is not able to determine if implied, verbal (for informational messages), or logged consent (could be a paper form) has been obtained.
Where this becomes important is if a customer marks your message as spam, or complains they did not want or opted out of messages and you continued to message them. In these cases you may be required to explain how you received consent, and in the case of promotional messages demonstrate the logged consent.
Later this year we will build in opt-out capabilities for our customers to help prevent sending messages to opted-out users, as well as to provide our customers with a list of numbers who have opted out, or opted back in via SMS keywords.
You can read more about how carriers classify these messages and the consent required on Page 12 of the CTIA guidelines: https://api.ctia.org/wp-content/uploads/2019/07/190719-CTIA-Messaging-Principles-and-Best-Practices-FINAL.pdf