General

There are certain ways to listen in on ones calls, they actually make devices that scan for cell phone frequencies and allow you to listen in on calls, or you can bug the phone, (just like a landline phone would be bugged.) There are also apps (viruses) that can forward all your text and voicemails to another number, generally to get this added to your phone the hacker would need to get physical possession of your phone, as far as I know though these applications generally do not allow you to listen in on ones actual phone conversation.

If someone beleives that they are a victim of such application/virus, they can either download a anti-virus to try to remove it, Try to manually find the program and delete it, or do a master reset.