Membership Signup Form - add "Phone Number" field (required)
To add to Rowan's answer, go to Administer>Customize data and Screens>Profiles and choose the profile that is being used in your Contribution Page.  On the right hand side of that screen you will see a button called "Fields".  On clicking that button you will be taken to a page called Profile Fields where there is another button called "Add Field".   Select the type of CiviCRM record and the field you want to include in this Profile ("Phone" is an option under "Contacts" and you can make it required).

Voila!  Hope it helps.