Can a private investigator tap your phone?
Landlines can be accessed and recorded as long as the investigator has the account holders permission. If not then recording devices can be deployed in proximity to the landline to record one side of the conversation. Modern-day smartphones cannot be tapped or 'bugged'. They use a digital encryption logarithm making it virtually impossible unless you're GCHQ and you have ministerial permission. Seeing as that pretty much rules out all of us and on the whole, most of us use our smartphones as our first choice device you should assume hacking phone calls is a no and any reputable investigator should not be including this strategy as part of your tasking plan.
