I think the best approach would be to combine HTML5 form attributes with an optional virtual keyboard link.
HTML5 form attributes can be used to trigger different types of keyboards. For example, <input type="email">, <input type="number"> and <input type="tel"> will trigger the appropriate keyboard types on iOS (not sure about Android/WinPho/other, but I would imagine it does the same), allowing the user to input the data more easily.
If you want, you could additionally offer a button to trigger a custom numpad under the text field for older non-HTML5 compliant mobile browsers. Those will display the new HTML5 fields as standard text fields.
You can use browser sniffing to detect mobile browsers, but don’t forget that those can still support things such as bluetooth keyboards. Sniffing additionally has the problem that it will almost certainly miss some browsers, and incorrectly detect others, thus you shouldn’t rely on it solely.