You can use the Length() function like so in a formula check box field.
Length will be 14 for a phone data type. It will be in this format (000) 000-0000.
You can make just a formula number field like this:
[formula number field]
You will likely get 14 as that is what a normal 10 digit phone number will return, 14 characters total.
Then use a form rule to see if this number is at least 14.