Wouldn't hurt to use the new SIM card if yours is over 8-12 months old. People fail to realize that SIM cards wear out over time, especially if you use your phone a lot. It doesn't cost anything to have it changed either, feel fortunate you got a SIM card in your box for free... they go for $20 at the T-Mo stores. Just call customer service and tell them you'd like to switch to a new SIM card that you already have, and they will ask for certain numbers from the card.