You asked for the activation phone number. I gave you a link to
a page containing the activation phone number. You were able to
contact Microsoft using the activation phone number at the link I
provided. I believe I correctly answered the question that you
asked.
Rebooting usually means shutting down and restarting the computer
by clicking on Start->Shutdown->Restart, or by pressing the
on/off button if Windows stops responding. Generally, rebooting
your computer does not require you to enter the Office product key.
When you install Office for the first time on a computer, or if
you uninstall and then reinstall Office, you will be prompted to
enter the product key.
It is not clear from your post exactly what you are doing or why,
so a better explanation from you would be helpful. It would also
help if you could quote the error message EXACTLY, without
interpretation or obfuscation or omission of the parts you don't
think are important. Did Office come preinstalled on your
computer, or did you purchase a retail copy of Office separately
from your computer? And although you say you have Office XP, are
you sure it's not actually Office 2003? If you really do have
Office XP, then were you able to activate the software at some
previous time using the product key that you currently have?
Please do NOT post your product key here if you reply to this
post.