I bought an unlocked T-mobile blackberry 8520 and my Fido sim card dosent work in it?

So, i got a T mobile phone on ebay and when i put my fido sim card in it it says invalid sim card. I know the sim card isn't in the wrong way because when i put it in another way it says insert sim card. (meaning that its not in/in the wrong way.) I don't know what to do and i need this phone to work asap! i thought maybe the phone wasn't unlocked but the seller said it was. Lets say the seller lied, how would i know if the phone was unlocked our not? Would it show ON the phone? Oh and without the sim card i can still use everything on the phone (except calling, texting, BBM , etc)
-I need all the help i can get ! THANKS! :)

You can use the phone with some other sim card and if it still throws the same error then, your phone is locked. I suggest you can approach Classicunlocking.com and specify the country,locked network of Blackberry 8520.Based on the availability you can get the permanent unlock service along with free instructions.