Phone number

There are a few ways you can limit the input field to accept numeric characters only.

<input type="text" onkeyup="this.value=this.value.replace(/[^\d]/,'')">

or a HTML5 solution:

<input type="number">

Sourced from: HTML Text Input allow only Numeric input

Or perhaps you are using the popular Contact Form 7 plugin, which has this solution to allow the use of numeric only input fields.

Hope this helps.