Iphone 3gs says 'no service'?

I just got another iPhone 3gs its exactly the same as the one I have previously. I'm on contract and with virgin however when I put my sim in the new phone it says no service, when I try my sim in my old phone it works perfectly. I've tried resetting the network setting but that didn't work. Why doesn't it work? Do I need to inform my contract that I've changed phone or do I need to do something else please help!

Added (1). The new phone is unlocked and there's good connection where I'm and I tried other sims from different networks and it still said 'no service'

Added (2). The new phone is unlocked and there's good connection where I'm and I tried other sims from different networks and it still said 'no service'

If you are 100 percent sure that the iphone you have actually works on the Virgin network? Then the phone itself may be blocked because of being reported lost or stolen.
I would suggest first trying another networks sim into the phone to see if it works? If this don't work then i'm afraid the phone is probably blocked! If it is there's noway at all to get it unblocked.
I recently bought a second hand phone which was fine for about a week then i got the same message.
And the person who sold me the phone had actually reported the phone as lost in order to get another phone so the network he uses blocked the phone totally.

Check to make sure your new phone isn't locked to another provider such as O2. I know with my old Iphone 4 (and with my iphone 5) if the phone is purchased through one company, lets say O2, and the phone is locked to o2 then the virgin sim won't work.

Means you got no service