do you own a myspace (If you do)
post a bulletin asking if someone has seen it if found please return or if you have any type of web space post a note. that is the best way to do so.
other way is impossible!!
I would call my service provider and explain that it is stolen then ask to have them track the calls-send you a list of calls made at the end of the month and take it in to the Police Station, then report it. If that isn't an option cancel the existing service and have them replace the phone if you have a warranty if not cancel the service until you can get a new phone so that you aren't paying someone else's bill.