Radio missions are missions given to you without talking physically with an NPC or object. You may receive missions both in warzones and instances, upon reaching a certain area, picking up an item or destroying an object, among others.
You can receive incoming communications by clicking on the radio icon that appears on the right hand side of your screen. When the mission is completed, the radio icon will appear again, letting you choose your reward (you receive any item rewards immediately).
Some missions may begin by talking to an NPC, and finish with a radio conversation, or vice-versa.