I was thinking about buying the Pantech Duo from ebay, but when I looked closer I noticed that there all AT&T 3G GSM phones. I want to use it with my existing T-Mobile account. So if buy one and put and my T-Mobile SIM would I be able to use the phone or would I need a AT&T 3G SIM. Would I lose features such as Text Messenging, internet or calling.