Is my gray market Sony Ericsson W810i locked?

I'm in India. I bought a Sony Ericsson W810i at a gray market shop not connected to any service provider. It was brand new still sealed in the original packaging. I took my SIM card out of my old phone, put it in the W810i, and it worked immediately. The phone also works here with SIM cards from other service providers. When I return to the States, will I be able to put my T-Mobile SIM card in and use the phone immediately or will I have to get it unlocked first?|||Nope your cellphone is not locked. The fact that you can put differents sim card on your phone and can pick up different service provider means it's unlocked. And yes it will work in the US, your cellphone is a quadband so it's fine. We have the same phone though.