Where I work we run a couple of high-security data centers, and the security policies don't allow outbound network connections to the Internet to be initiated from inside the datacenter. It's a good policy and makes for a much more secure environment. So, when it comes time to activate a copy of Windows Server 2003, I frequently get asked how to do that over the phone.

I could just say "Ask Google," but instead I think I'll just point people here, heheh...

The Microsoft Windows Product Activation phone number (for the US anyhow) is 1-888-571-2048

Also -- It's worth noting that Windows should tell you what number to call if you let it. From the Microsoft web page on the topic:

*** Toll-free telephone numbers are available in all countries where telephony infrastructures provide for them. The telephone numbers are displayed when telephone activation is chosen.*