This site has thousands of posters and readers who find the advice quite satisfactory.
SomeoneSomewhere is quite right to say that if you didn't have any security software installed on the phone before you lost it or had it stolen. All you can do is to notify the service provider so you don't get charged for any calls made by the finder or the thief.
If the phone was an Android device and you buy another one, go to the Store and download Lookout. Also back up all the personal files to a PC regularly in case of failure or loss.