Hyde here from Shopify.
I assume that you've set up your store to offer customers the option to receive SMS right? That's all outlined here.
And this is, of course, how to test it out yourself:
If you're trying all this, without success, it might be a question of checking whether the different numbers are using the same mobile provider? If not, then it may be a technical issue we need to delve into with the technical team.
Let me know and I will reach out to you by email about this!
All the best, Hyde.