I think this is a nice suggestion, sure, but it'd be basically putting people and some certain IC items out of question. You can already do all this, just not through a phone line.
|-||zeezenfrozen||1m||stay goofin', gaffin', always laffin'|
|-||Ryuzaki4Days||18s||Take drugs. Kill a bear.|
|-||ReeferMadness||1h||May the bridges I burn light the way.|
|And 18 more hiding and/or disguised|
I'm not saying it's not a good thought, questionable if it's more realistic (eg, maybe future phones have incredible noise filtering technology for just this reason), but I just think it would be annoying to have to worry about.
Or maybe this is a separate item that can be slotted into a phone to accomplish the same thing. Being able to bug someone else's phone would be very OP but would also give secure techs another move. You want to create true paranoia, worrying that someone dipped your phone, walked a room over, stuck a bug on it, then planted it back in your pocket without you realizing...
Not that you'd plan on robbing or murdering someone IRL, but would you REALLY be on the phone with your target while you're talking to someone else about doing that exact thing? It seems like a huge risk. "Oops, I forgot to mute the phone. Now Mark knows we're trying to set him up. Back to the drawing board."