So, this is actually kind of cool - Your smart phone has a "phone" function where you can dial a "phone number" which, if you're in the US, is 10 digits long, and it will send a signal over your mobile network and 'ring' the other phone of the person you're intending to dial, and, if they answer, you can speak with them directly and in real-time. And if they don't answer your call, it will go to an automated message where you can leave them a "voice message" (kind of like sending a voice text) where they can listen to it later if they feel like it.