How to spy on a phone without installing software?

Can you spy on a phone without installing software? It depends on what exactly you want to know about the phone you’re spying on. If you want to find out the GPS location, then Google and Apple’s free online services are there. Also, particular mobile providers such as AT&T allow tracking the GPS location and SMS within their programs.
If you’re willing to track instant messengers, calls, online activities, then spyware is a way out. It has to be installed on a target device. Further on, physical access is not necessary.