I agree that this approach is actually pretty reasonable for checking if a specific string is in a relatively small list of possible values that can't easily be

I agree that this approach is actually pretty reasonable for checking if a specific string is in a relatively small list of possible values that can't easily be expressed as a simple set of prefixes or whatever. It's must simpler and easier to maintain. The bigger problem with this approach is that it exposes a canonical list of phone numbers that are guaranteed to not be flagged as fake by the tool itself. That isn't likely to be a _massive_ concern here specifically, since it will always be easy to find a valid customer service number without that access. But I'd expect a team looking at dealing with fraud or scams to be pretty conservative about information disclosure generally – it would be a good argument to implement this as a backend service.