Unlocked iphone 3g + sim problems

I bought an unlocked iphone yesterday from vodafone greece. The phone is sold here without carrier locks and can be used with any company. i ve put my almost 8 year old sim (which worked without any problems in lots of other phones) and i got a no sim error. My mate has bought an iphone before a few days and i took his sim that worked on his iphone (vodafone sim) and again there was a no sim error. I went back to the store and they put a random vodafone sim from there and it worked fine. Their advice was that maybe my sim was old and it needed replacement. I transfered my number from another carrier to vodafone and went today to pickup the new sim.
As i work in the uk, during last night i tried my tmobile sim and again there was a no sim error. My girlfriends sim from the same carrier as my original greek one worked fine. The new vodafone sim that i got today works fine, but since it is not activated yet, i am using my old htc kaiser with my old sim. I tried another sim from a friend and again there was a no sim error.
Tmobile will send me a brand new 3g sim till i go back to the uk, but i am worried that it might not work as well.

If the other carrier sim wasnt working i would think that i just got a locked device by mistake, but since that wasnt the deal, i am worried that i might have a dysfunctional iphone as the other 3g vodafone sim didnt work either.

Keep in mind that i am not talking about any "jungle" unlock methods, as the device is sold unlocked here in greece.