iPhone 3Gs waiting for activation problem

Hi all,

I bought an iphone 3Gs from ebay...
The phone has been said to be unlocked...
But when i put my sim card there is this message: ;waiting for activation - this may take some time;....
i tried both O2 card and 3 mobile...

So what's wrong? The seller has told me that it is definetely ulocked cause he used to have it with an O2 sim and later with a T- Mobile..
He then clearred off all the info and sent it to me...

What is wrong and what should i do?