According to http://www.ehow.com, to assist people in finding a lost device, both Google and Apple have included ways to track a missing phone in their respective operating systems.
The features are in-built, and here are procedures to set up, access them and use them to track your lost devices.
Finding a lost iPhone
Apple’s solution is called Find My iPhone. This same service is capable of tracking any and all of your iOS devices (as well as any Mac associated with your Apple ID). To enable it on your iPhone, follow the steps below:
Launch the Settings app on your iPhone
Scroll down and tap on “iCloud.”
Near the bottom of the list, find and select “Find My iPhone.”
Slide the switch next to Find My iPhone to the On position.
While you’re there, you can equally turn on Send Last Location.
With Find My iPhone enabled, you can track a lost or stolen device through the Find My iPhone app on another iOS device or by visiting iCloud.com and signing in to your iCloud account. When tracking a device from either the app or iCloud website, you can lock it with a new passcode, track where the device currently is as well as where it has been, and as a last resort, you can fully erase all contents of the device.
Note that your iPhone will need to be turned on and connected to the Internet in order for the service to work. So, if a would-be thief picks up your device and immediately turns it off, you’re out of luck until it’s turned back on and able to connect to a network. If Send Last Location is enabled, your iPhone will report its last known location as the battery gets low. This won’t help you pinpoint the exact spot of the device if it’s been moved after the battery dies, but it will provide you with a good starting point to begin tracking your device.
In order to disable Find My iPhone on a device, Apple requires the user to approve the change with his or her Apple ID password. In other words, should a thief pick up your device they will be unable to disable Find My iPhone altogether unless they also know your password.
Finding a lost Android phone
Google’s Android Device Manager works more or less the same way as Apple’s Find My iPhone. Instead of having the service pre-installed, however, you will need to download the app from the Play Store and set it up in order to use it. Here’s how:
Search the Play Store for “Android Device Manager.”
Install the app and then sign in with your Google account.
Be sure to leave the checkbox “Never Ask Me Again” unchecked. If you leave that box checked, anyone with access to your device will have the ability to disable the feature or track your other devices.
You can also use another Android device to track your phone using the app, or visit Google’s Android Device Manager Website. You’ll need to log into the site using the same Google account you used to sign into the app on your phone.
Using the app, you can play a sound on the device, lock it, or completely erase it. As with the iPhone, if the device is powered off, you will be unable to track it until it’s turned back on and connected to a network.