User input from Control panel outputted to template files?

I’m developing a series of child themes from one master parent for a client who has 5 sites. Each of the sites has a different telephone number as a point of contact.
I need to display the number in the sidebar, footer and other places through out each site.

Is there a way to create a “holding element” for the number in the template files and pass the number to it from the Control Panel?

This way the client can update the phone number via the Control Panel as and when they need without having to change template files.