I am a complete newbie but I have a different kind of suggestion on how to receive SMS text messages (ST notifications) at more than 1 cell number, for those of us who want a simple solution. We can setup more than one recipe of Notify Me When - make them identical but enter a different cell number into each one. Here are details steps:
SmartThings Classic > Marketplace > SmartApps (upper-right) > Safety & Security > Notify Me When > choose one or more events. Where it says “Via a push notification and/or an SMS message”: If you leave that blank, you’ll receive a push notification … scroll down to choose “Notify me via Push Notification” and choose Yes. If you enter a cell phone number, that number will get an SMS message. Click “Save”. (upper-right) You won’t be able to enter more than one cell phone number here either, but here’s the cool thing. You can setup multiple Notify Me When ‘recipes’. So just create the same recipe over again, with a slightly different name. Then, two different cell numbers can get the same notifications.
To view your Notify Me When recipes: Choose ‘Automation’ in the lower bar > SmartApps (upper-right). You can edit or remove them any time. To edit one, click on it, make changes, choose Save. To remove one, click on it, scroll all the way down and choose the red Remove button.
I hope this helps someone.