According to the documentation setting this in Checkout settings:

should disable SMS messaging. However it doesn’t, Shopify continues to send messages:

A different post: https://help.shopify.com/en/manual/orders/notifications/sms-notifications
seems to imply that Shopify will send a SMS if it finds a phone number and ignores other settings?
I found some other things that suggested there’s a flag in the customer contact info to disable SMS messaging but I can not find any.
I want to shut of SMS for all customers unless they specifically request it when they place an order. This includes orders entered manually. I thought that was what this option did:

i.e. they would have to specifically opt-in to get a SMS. If allowing them to select it means they get an SMS regardless of the selection or it’s defaulted to ON and no way to disable it then I would just disable this option and presumably then the system would only send emails?
We require both a phone number and email address to process an order.
Where’s the OFF switch for SMS?
Thanks,
Brewster