Use ng-pattern, in this example you can validate a simple patern with 10 numbers, when the patern is not matched ,the message is show and the button is disabled.
Also you can use another complex patern like
^+?\d{1,3}?[- .]?(?(?:\d{2,3}))?[- .]?\d\d\d[- .]?\d\d\d\d$
, for more complex phone numbers