1. Take an advantage of HTML5 validation
<input type="tel" name="phone" required=""></input>http://diveintohtml5.info/forms.html
2. Use the fallback where not supported http://afarkas.github.com/webshim/demos/index.html
] or similar
3. Always to server side validation in addition. It is not quite clear what language you use, but any server side language has some validation libraries.