Keep info in one place to be use in several locations
There really isn't a best practice. I'd say it depends on the intent and what part of Drupal's CMS functions best fit your scenario. For example:


Is it a part of distributed theme template? It could be a theme settings.
Is it a part of functional application logic? It could be a variable added to a site configuration form.


For most practical purposes of boilerplate phone info in a footer, I'd think most people would go for block with a region placed in the theme's footer, but Drupal is flexible by design so you could see alternative decisions depending on the intent.