On the AjaxRay.com site today there's a new tutorial for the Zend Framework users out there with a library they can use to extend Zend_Form for custom phone number fields.

When taking Phone number as user input, we can worn users about phone number format by setting a hint/description and can validate using Regular Expression. [...] Now, if we try provide this feature in Zend Form, that's possible. We can create three individual Zend_Form_Element_Text objects and join there value together to make the phone number. But, in this case, validating them together is a hassle.

Instead of separate fields, the library they create makes it simple to handle them as a whole field. It works as a helper for Zend_Form and lets you set things like the separator between the text fields, a "format" string and a validator to apply to their fields (in the example code, it's the "digits" validator). Sample code is included to show you how it fits in your form.