How to spy on people’s phones?

To spy on people’s phones depends on the type of information you want to know. If the GPS location is the subject of interest, then Google and Apple location services, IMEI number option come in handy. If a person needs to track text messages, then services within mobile providers make sense (AT&T, Verizon). To spy on calls, Viber, WhatsApp or any other instant messenger, online activities, keywords, spyware is indispensable.