Once upon a time, phones had dials on them, and for someone to listen in on your conversation, a physical connection had to be made on your phone lines. With VOIP, someone can listen in to your conversation from the other side of the world, never physically touching your hardware.
How is this done? It’s called “sniffing.” Your VOIP call’s sound is converted from the sound vibrations in the air to digital bits sent over the wire. You may think that’s not a problem, because it’s supposed to be a straight shot from here to there. However, there’s ways to reroute those packets to a special vantage point, where someone can monitor, and reassemble those packets back into audio without you knowing about it. Believe it or not, this isn’t even “hacking,” because your conversation is like a bunch of post cards floating through the air, instead of being inside sealed envelopes. Those post cards can be read by anybody.
So how do you work around that? It’s an easy thing called “encryption.” It works like a super decoder ring between you and the other party, to scramble those digital bits and keep other people from eavesdropping. Teo phones support TLS/SRTP encryption, which keeps everybody from listening in. Instead of your conversation being written on post cards, now it’s in sealed envelopes, and what’s inside is scrambled. Now nobody can listen in, and your conversation is safe and secure.