Traveler communications devices, called comms for short, are tiny graphene-based machines powered by the energy in the human body. (S2E01: Ave Machina) Traveler teams implant the devices in their necks, which allow them to speak to each other across great distances with little interference. Comms are only used for intra-team communication; inter-team communication is generally done through conventional means or via the deep web.
Comms can be activated or deactivated manually or by voice command. Certain finger press patterns denote the nature of operation in manual mode. Comm communication is encrypted and has an effective range measured in miles. (S1E01: Travelers, S2E01: Ave Machina) They also provide GPS tracking. (S1E05: Room 101) It is also possible to disrupt another person's comm using 21st century technology: the FBI has developed a device that emits loud feedback only heard by people with active comms. (S3E10: Protocol Omega)
The devices are implanted via injector. Taking them out, however, requires cutting through flesh to physically remove the device, which can cause copious bleeding if done improperly. (S2E08: Traveler 0027, S2E11: Simon)
Traveler 001 has a means to overload a comm while still embedded in a user: it glows hot, causing great pain as it falls out and cauterizes the wound. The procedure how is unknown to other Travelers. (S3E01: Ilsa)
As electronic devices, comms can be used to "stream" nonvoice data like music over the wireless connection. (S2E11: Simon)