Detailed info for phone validation is on this page: How to contact a Microsoft Product Activation Center by telephone, but essentially as Phil described:
To start the Product Activation Wizard, follow these steps:
Click Start.
Click All Programs, and then click Accessories.
Click System Tools.
Click Activate Windows.
In the Product Activation Wizard, follow these steps:
Click Use the automated phone system to activate.
Click the drop-down menu, select the location that is nearest to you, and then click Next.
The Product Activation Center telephone number is displayed at the top of the window.
I used it once years ago after I changed a motherboard - which I think is strictly not allowed - and they validated my old copy for me with no trouble.
Mick