Hey David,
The forms 2.0 functionality should natively prevent non-numeric values from being entered on your phone number fields. As for fake or dummy numbers, I don't think there's a great way to do this on the form-end, but you could build a smart campaign to watch for dummy/invalid data coming through the phone number field, and have it null out the field when it finds a match. The only problem with this is you'd need to account for a lot of different situations in your smart list filters. For example:
Phone number starts with: 12345
Phone number is 555-555-5555
Phone number is 867-5309
and so on.
It won't cover every circumstance, but it should help keep some of the crap out of your system. You could even build an initial scrubber with some of the values you've seen so far, and then have it run for a few months, checking back to see what it might have missed, and then add those missed number combinations to your filter to strengthen it further.
Just an idea!
Hope this helps,
-Jeff