The most obvious way to locate a lost cell phone is to call or text the phone and, if it has been found, arrange for its return (a reward might ease the process). There are also several ways that technology can help, through GPS tracking or various mobile applications.
If calling or texting the phone did not help to locate it, and retracing steps did not work, it is prudent to call the phone's service provider. They may offer a GPS locating service to help find the phone. If they do not, have them suspend service to the phone in order to protect yourself from charges and fraud. Afterwards, use the phone's serial number to register it as lost at a website such as MissingPhones.org. Tell your contacts that your phone is missing, so they know to contact you by other means.
If the lost phone is a smart phone, certain mobile applications may be used to help locate it. Android users can use the Plan B app on a computer to help them pinpoint phone location. Apple users can download the Find My iPhone on a friend's phone or tablet and locate the phone, lock it, erase its data or force it to make noise that will make it easier to locate.