A communications device, in the context of the PPC, is any one of several technological devices carried by agents to maintain voice contact with Headquarters in case of emergencies, as well as with each other at a distance (provided the canon's tech level will allow it). Colloquially, they are sometimes called "comms devices," or just referred to as "comms" for short (do note that, due to regional differences or simple preference, some people will always use "comm" rather than "comms").
Though there are exceptions, most devices cannot as a rule manage cross-world contact, and as such have to be modified with KI technology by the Department of Sufficiently Advanced Technology before they can be properly used.
If in a world with pre-World-War-II levels of technology (as that was the rough time period when handheld two-way radios began to appear), agents are advised to refrain from using their comms devices altogether except for contacting Headquarters in emergencies.
Technological Comms Devices
A mobile phone, commonly referred to as a cellular or cell phone, is a wireless telephone that typically operates over a cellular network of broadcasting and relay antennae. Though popular on World One, these phones will not normally work except on world One or in other places with similar intricate antenna networks. If modified by DoSAT (or the Doctor), on the other hand, these phones become capable of working practically anywhere in the multiverse thanks to the PPC's plothole technology.
A comlink is a communications device from the Star Wars continuum, used to transmit voice signals much like a cellphone or two-way radio. There are numerous models of varying sophistication and power; however, as a rule, they are usually smaller than cell phones or walkie-talkies. Comlinks are often clipped onto headsets for hands-free communication, either out of preference or necessity (such as between starfighter pilots). Some forms can even be implanted under the skin, though this is quite uncommon. Without satellite amplification, however, comlink signals are relatively short-range, and this range decreases even further in urban environments. Hence, DoSAT modifications are required for comlinks to be usable as proper comms devices.
KIs themselves technically count as comms devices, being capable of voice contact and cross-world communication. However, the KI's design is rather eye-catching compared to other, more subtle devices, and depending on who you ask, this has made it either more popular or less popular than alternative forms.
A two-way radio is a radio set that contains both a transmitter and a receiver, making it capable of both sending and receiving radio-wave messages. This can technically refer to any set of any size, but in the PPC it usually refers to the handheld variant, commonly known colloquially as a walkie-talkie. Infrastructure Departments and Security Departments tend to use walkie-talkies and a private channel for in-HQ coordination, but agents have been known to use modified walkie-talkies as their preferred comms devices. This is mostly in canons where such devices are commonplace; usually World One canons, and especially crime shows. Walkie-talkies used by agents as comms devices have been modified by the DoSAT to use plothole technology, so that they can contact HQ from anywhere in the multiverse.
Organic Comms Devices
Den Den Mushi
The Den Den Mushi (always capitalized) is a species of snail from the One Piece world that is currently being investigated by DoSAT's Testing & Applications Division as a possible organic alternative to traditional comms devices. Den Den Mushi have the natural ability to communicate with each other at range, as their brains are capable of sending and receiving radio signals. By attaching a microphone and a number-button panel to these snails, it is possible to use them for long-range communication (the snail's mouth acts as the speaker).
The problem DoSAT is currently trying to overcome is portability. A Den Den Mushi's transmission power depends on its size; the larger the Den Den Mushi, the longer the range, and the less easy to transport. "Baby Den Den Mushi," a smaller species, do exist and are used in the One Piece world as portable communicators, but their range is significantly less, and T&A is still trying to figure out a way to increase the range and gain cross-world capability without seriously harming the specimen used.