The Android Dev Phone 1

you could also ask your mobile provider for a secondary SIM, some do it (if you're lucky only for a one-time fee to pay for the card). The beauty of this is that both phones are logged into the network, both ring when your main number is rung, both send out the main number when calling or texting, but the new SIM can also be called on it's own number. I find this easier and cheaper than getting an new SIM with a separate number.