I have found this: https://support.ringcentral.com/s/article/RingCentral-Business-SMS-Frequently-Asked-Questions?language=en_US
which says that I can do it through the phone app - but we use the Glip App not the Phone App (which I believe you are supposed to be integrating and removing the old phone app?)
How do I do this? And can I select which of the company numbers the message is sent from?
UK account, 200 users.