A hail was a form of subspace communication between starships, starbases, and other ports of call. It was in use since at least the mid-22nd century and could be digital (computerized messages), audio, or visual.
Most hails were visual, although routine operations could be handled by the computer or communications officer without requiring the commanding officer's involvement. In addition, hails could be audio-only. The treaty ending the Earth-Romulan War was negotiated entirely over audio channels, as neither side had the technology to send visual images to the other. (TOS: "Balance of Terror")
A hail, whether audio, visual, or computerized, involved two-way communications. A hail was sent by one party, and needed to be responded to by the other to initiate communications. When a recipient was unwilling to accept a hail, a one-way wide-band subspace transmission could be initiated by "opening a channel," although it was customary to answer all hails. Such a message was usually not directed at any specific target, and there was no guarantee that it would not be intercepted even if it was earmarked for a specific recipient. In fact, there was no guarantee that it would be received at all. However, it could be reasonably assumed that any race that possessed the technology to receive these messages would probably view or listen to them out of curiosity, if nothing else. If needed, the receiving party could escalate the communication into a full two-way conversation by starting a return transmission. (citation needed • edit)
A hail requires communications technology. The Vulcan monks at the monastery at P'Jem couldn't be hailed before the landing party from Enterprise NX-01 transported down because they were lacking the technology. (ENT: "The Andorian Incident")
Types of hail
Hailing did not require direct communication with the other party; messages relating to routine procedure or an unimportant matter could be handled via computer signals. For example, when Jem'Hadar attack ships requested ketracel-white from Dominion installations, the procedure was conducted entirely through computerized hails. It was more common for a computerized message to precede visual contact, however. (DS9: "A Time to Stand")
When dealing with a vessel or location whose hailing protocols or capabilities were unknown, a general message could be sent over wide-band subspace. Additionally, wide-band subspace could be a technique for contacting anyone in the vicinity during dire situations. (citation needed • edit)
The most common type of hail was through audio or visual contact. The recipient had to first accept the transmission in order to initiate such contact.
Making visual contact was normally standard, but it was not required. An audio link could be established when either a visual link was not possible or one did not wish to be seen. Some foreign entities were also incapable of a visual link and thus communicated over audio channels.
The more common method involved a two-dimensional display showing the party on the other end. The area displaying the image was known as the viewscreen. One could pause the hail, which briefly stopped the transmission, as well as mute the other side when necessary. (citation needed • edit)
Holo-communicators were a new type of communications device used briefly by the United Federation of Planets circa 2373. They utilized holographic technology to create a life-size version of the person on the other end of the hail. As Captain Benjamin Sisko said to Captain Sanders when he first used a holo-communicator, "It looks like you're on my bridge!" (DS9: "For the Uniform")
When making visual contact, a person could often use a holo-filter to mask their appearance. However, if the transmitting ship was scanned at close range, sensors could alert the recipients to the person's true identity. (DS9: "Second Skin")