not a really good advice .. phone ID is linked to your SIM id in the network databases ... no problem to find your previous sim's from your current new SIM (but still using old phone) .. never do it!
OK then let me re-phrase this just for you:
Get an UNLOCKED GSM (AT&T, T-Mobile) phone and buy any SIM card from any of the carriers supporting GSM standard and you'll be fine.
Or do what this guy does: