I wondered this myself actually, but I think you would need to know the person to know the phone number you want to track. iPhone has various apps that will let you do that but in an attempt to locate my daughter with them I found they are for entertainment purposes only and only track the app users phone..kind of disappointing. So if anyone knows an app that actually works I’d like to know, as it would be a great way to “secretly” keep tabs on my teen! Unfortunatley that’s all I can offer, maybe you @yoyoo29 can search the apps and let me know if I have missed the right one? Sorry I wasn’t much help….