NEED IPHONE 3GS Help, ACTIVATION UNAVAILABLE?

Basically i have a second hand IPHONE 3GS which the previous owner had on giff gaff, i bought a giff gaff sim and put it in my phone after i reset it, i pass the language and country setup, i choose my Wi-Fi and then start to activate my phone, it leaves it about 2 minutes and then gives me the message:

error message activation server temporarily unavailable.can't activate iphone

I KNOW FOR A FACT IT WASN't JAIL BROKEN So SOMEONE Please Help
iPhone 3GS, I Can't ACCESS MY PHONE

Reformat it.