Hi
When you purchased the phone from amazon did it state unlocked
There are some tell-tale signs of a locked phone:
The phone packaging/box has your operator’s logo on
The phone itself is branded with the operator’s logo
You see your operator’s logo every time you turn on your phone
There are items relating to your operator in the main menu (e.g. Vodafone 360, Orange World)… although sometimes you’ll see the branding even on a phone which isn’t locked
Was the phone purchased from an overseas market
Foreign networks are treated as totally separate networks for the purposes of locking. This is true even if the foreign network shares it’s name with a UK counterpart… e.g. if you have a phone locked to 3 Hong Kong, Orange France, T-Mobile Germany, Vodafone Australia or so on, the phone won’t work on the British counterpart network.
Best I can do at moment suggest revisit amazon page or even contact them for sales advice....