The most powerful and resultful way to spy on cell phones is using spyware. It is supposed to be set up on a target device and run in the background. That way, it is invisible for cell phones’ owners and meantime can record and pass the information to another person.
Spyware is service-based and suggests subscription plans and packages with features (e.g., monitoring of SMS or Viber, WhatsApp, Snapchat, etc.). At present, these apps are the most demanded: Highster Mobile, mSpy, FoneMonitor, SpyMyFone, Spyzie, SpyBubble.