There are plenty of apps for both iPhones and Androids that can help you locate a lost phone, but they typically require you to set them up on the phone in advance. Obviously, that won't be much help to you if you’ve already lost your phone! Your phone’s cellular carrier might be able to help you track your phone down using your phone’s location services, but that typically takes a while (and it doesn’t always work).
Luckily, there’s one method that almost always works if you’ve simply misplaced your phone and it happens to be within earshot. Simply visit CallMyLostPhone.com and type in your lost phone’s number. The website will place a call to your lost phone, allowing you to track down its hiding place as you listen to its ring. The service is free, and it works just as well as calling your lost phone from another telephone.