Phone number validation

We use a phone field to input phone number for our forms which has a SFDC sync, but there doesn't seem to be an easy way to validate the data. We've noticed people being able to put in 1 in the phone number or 123. Just looking to not allow people to put less than 8 numbers in the phone field (but could be more than 8), and only numbers. Is there a way to do this without Javascript? I've seen the instructions on masking but this seems to be only if you want it to be exact 8 not 8 or more.