When a cell phone "speed dial" button is pushed inadvertently, and yours is the phone dialed, you pick up and hear any conversation happening near that one, inadvertently dialed cell phone. It's not a two-way TELEPHONE converstaion, it's a converstation taking place between two people in the vicinity of the inadvertently dialed cell phone!
Even if totally false, the scary thing is that it is all plausible.
Stay vigilent, and stay armed!!
So not only does your phone have to dial another number, the person on the other end must not only have their phone on, and be near it, but be talking on a THIRD phone?