How can I validate a phone number with a workflow?

We want to make sure phone numbers entered into the site contact phone field don't use special characters and contain 11 digits, any idea how I can achieve this?

The only way I've thought to do this would be by checking if the number is within a certain value of 11 and 10 digits. But that doesn't appear to work because with the field being a phone field it turns 12345678910 into 1 234 567 8910 and then that messes up the check.
Should I try to stop the field from auto formatting and how would I do that? Or is there another option I'm not considering?

I don't want to script this, I'm currently struggling for script space as we have 12 CS scripts on SOs from bundles alone.