You would not know your phone was being tapped. If done legally by law enforcement then I guess you would learn about it in court afterward. The call would be intercepted passively at the telco's switch, you would not hear any *clic* on the line as portrayed in movies.
Even those off-the-shelves Harris-Dracon buttsets does not make any suspicious noise when clipped to the line, I've tried it, however I have no clue
if this would tip off a voltage fluctuation detector.
Your cell phone can be tapped at the switch level as usual by law enforcement. Or a clever eavesdropper could intercept the radio signal with a
scanner. GSM strongest A5/1 encryption can be broken with a personal computer. I don't know about iDEN or CDMA protocols, but chances are those
algorithms are as weak as GSM.
As for VoIP, there already are a couples of Linux softwares floating around on the internet to intercept that thing.
Be afraid, very afraid! *Cue the Alfred Hitchcock's sound effect*