Not all XP's are the same.
It could be that you have a CD for XP Home, and their Serial is for Media Center or Pro... For example.
I also know that some keys also dont like Service packs either... Again and example of this, is that I have one XP Home Licence and it hates it when I try to install a SP2/SP3 CD and it wont allow it, so, I got around it by naughty means.
And then there is the other ones where certain keys are OEM keys and only work ( apparently ) with the correct OEM installation disks ?
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Either way, you seem to have got it installed, so the installation procedure has obviously accepted your licence and it is therefore corect for the type of XP you are using.
So, if it fails to activate over the phone, simply chose the option to speak to someone and they will walk you through getting it activated.
I have had to do that loads of times... As long as you are only using that licence on one PC then you will be fine.