The following tutorial explains how to add telephone number to Chameleon Theme footer on a Plone 4 website. Instruction for Plone 3 website - see in Footer Customization in Plone 3 tutorial.
Chameleon Theme footer is subdivided into three parts. Telephone number can be displayed in the footer left part, area for which is reserved in default Chameleon Theme.
To add Telephone number to the footer left part - create a page with telephone-number shortname in the root of the site. Type in necessary telephone number into the page description and save it. Now this telephone number will be displayed on all site pages.
Footer center and right parts content is taken from footer.pt template that can be reached via ZMI at portal_view_customizations -> chameleon.footer (quintagroup.theme.chameleon.browser.interfaces.IThemeSpecific). Just customize it according to your needs.