I actually just figured this out! You need to create a joined list column with the phone numbers you want for a given text, with the numbers separated simply by a comma (no spaces). Then create a template column where you add in the following url to the start: sms://open?addresses=. I noticed that the number needs to be formatted XXX-XXX-XXXX; without the dashes it doesn’t work. (not sure if it handles parentheses or country codes).
So the data in your column would look like sms://open?addresses=555-555-1234,555-555-5678,555-555-9012. Then in your button, create an Open Link action on the button, and the link should be that sms://open?addresses=etc column.
This works in iOS. I wound up creating a second button for Android that uses the Compose SMS action pulling just from the joined list (without the sms::// url). Not sure how (or if it’s possible) to combine the buttons and make them conditional on the user’s device; I’m assuming not possible.