We want to users to ONLY use a mobile/cell number when completing a form - and the form JS to reject land line numbers. Thi is so we can use SMS automations for follow-ups. As mobile/cell formats will be different in different countries, this could be a simple setting that allows us to require the first "X" digits to be "Y" numbers.

So, in Australia, we could set this as - "the first 2 digits must be 04"... with a custom error message eg. "Mobile number required" (I know this can be done by custom JS, but it's painful...)