How to spy on someone’s messages for free?

One might spy on someone’s messages for free if their phone and the target one belong to the same mobile provider. Also, they must be subscribed for the program that comprehends a few participants in one. For example, for Verizon phones, there’s the option Smart Family where family members are linked and can read each other’s messages.
Another option – Google Voice. Android phone number must be associated with a Google Voice one. Once someone sends a text message, Google Voice makes is visible on its account.
Any other ways to spy on someone’s messages like spyware is paid.