The use of nanomachines to secretly communicate among one another. In order to use this high-tech device, one must have pre-implanted nanomachines in their body. Nano-communication sends nerve impulses through your body until it reaches your brain, where you can hear the other person's voice. This technology was invented in 2027 AD, near the start of the third world war and was mainly used by spies investigating the development of Metal Gear Rex.
"Snake, switch over to nano communication. Someone may be listening in."