In order to find a lost mobile phone, you should first narrow down the possible areas where the phone could be located, especially if it is misplaced in your home. If you still cannot locate the lost phone, trying to call the phone from another one and listening out for the phone's ringtone would be wise especially if the phone can be in the vicinity.
However, trying to locate a stolen cell phone which was lost somewhere else other than at home might require a more technical approach. You could inform the police and your service provider to help you locate the mobile phone. One should however ensure that the telephone is not within ones area of operation such as at home or the office. You should also be having all documents related to your phone and disclose the required info to the service provider and they should be able to update you if someone else uses your SIM. Some are even able to inform you if the person in possession of your phone uses a different SIM in your phone. Also get more information related to this topic, check - IMEI Tracker Online and Track IMEI Number Online.
GPS could be used to find a lost mobile phone if it is GPS enabled. The GPS technology enables one to pinpoint the exact location of the phone, through the internet. This would however, required the owner to have installed tracking software on the phone as it would be quite difficult without the tracking software. Smartphones are also able to be located using various internet applications, with certain applications even sending a message to the phone. The message could contain a way of returning the phone to the original owner since its place is already known.