Ok. You need to report the phone to the police immediately and get a police report number. Then you need to call your carrier who will deactivate it.
Because you are liable for any and all calls made on the phone until it reported to the carrier and they shut it down.
Unfortunately, when the thief inserts his SIM card, he will be able to use it on his carrier - unless of course it is the same as yours.
There's not much else you can do. Sorry.
Next time, consider installing software like PhoneBak. Then, if the phone is stolen, it will contact you with the thief's phone number.