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
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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.