disallow non-digit characters in phone numbers

[shaeespring]

Use regex to remove strings from a phone number and don't save the phone number if it's less than 9 digits or greater than 15

[costowell]

Overall, looks good

Some things you can ignore if you don't care:
I don't think its necessary to accommodate anything that isn't a 10 digit number (123-456-7890). Only reason why I mention that is because it'd be nice to format the phone number nicely after removing all the junk from it (i.e. add the dashes back). Completely up to you though, I think its doing fine as is.