While there are no definitive ways to know if someone is “tapping” your phone, there are a few things you might think about. Keep in mind cell phone companies could always be the ones who have tapped it especially if they want to sell you more or get ahead of any competition.
Monitor outgoing calls and texts – If an owner consistently makes calls to the same number(s) and texts out only to similar numbers on a regular basis then that’s still not proof but should make him/her raise their eyebrows. For instance, a hospital would call all patients every morning at 8am, but this would not feel suspicious because the entire city or state has been conditioned to expect this call from that hospital for years.