Why my Blackberry Curve has GSM coverage?

I bought my sister unlocked Blackberry Curve about a month ago and for the last 3 and a half weeks its bin showing that i have EDGE coverage. About three days ago I got GSM coverage. I looked it up and it says that its the cheapest coverage. My plan has 3G and im able to send a recieve pictures and now i cant. When i put my sim card in my old phone it sows 3G and on the blackberry it doest. Ive taken out the batter, reset the entire phone to factory OS, and also replaced the sim card. anything else i can do to fix this or is this just not fixable? I also looked up that maybe a tower is down but why would it show GSM on the blackberry and 3G on my solstice. Ive also put my dads sim card in and it showed edge