I agree. I activated XP with a downgrade from Vista Business just this week on that number.
Running away from Vista?
btw I also have activated a couple of Winders licences via telephone - and it worked just fine for me.
For those who've never done it, the procedure is as follows :
First you click on "Activate this copy of Windows" - and tell the program that you'll want to phone a consultant. It will then generate a couple of numerical digits in 7 (or 8) groups.
You then either read the numbers back to a human consultant, or you punch them in on the telephone's keyboard for a computerized consultant.
With each group of numbers it is verified that which you've entered is correct.
Then after all the groups have been entered, another group of codes is generated on M$'s side, and the consultant (or computer) will start to read it back to you. There's no need to panic here if you miss a number, you get the chance to repeat the lot, until you have entered all the groups. The numbers is all numerical again.
Once all the numbers have been entered into their correct groups, your copy of Windows should be activated. I don't know what happens next if it doesn't get activated beyond this stage, it always worked for me.
Be prepared beforehand, choose a quiet place, within reach of the computer.