You really should be able to send SMS messages to guests. This very basic and available in PMS systems far less advanced than mews.
Should be top priority
Hello from the Mews product team. We are happy to let you know that your feature request has been released. We've introduced SMS to Mews, and it can now be set up through the Marketplace. For more information, please check out our release note here: https://releases.mews.com/en/improve-online-check_ins-and-increase-upsell-revenue-with-our-sms-add_on-in-mews-marketplace-iFxmojhC. Thank you once again for sharing your idea!
Bartomeu Agustí Ferré commented
BOOKING.COM is currently blocking any email message that we send from MEWS to our guests. So no guest journey for those guests who booked via BOOKING.COM. Would be convenient that hotel could start up the guest journey by SMS.