Cable telephony is the use of a cable television connection as a means of transmitting telephone calls. It uses the local cable network to send and receive data in a similar manner to a cable Internet connection, though cable telephony does not itself use the Internet and so is distinct from Internet telephony. Due to the ability to compress data sent over cable lines, cable telephony can provide more efficient use of bandwidth than conventional telephones, though it also suffers from reliability issues not faced by conventional land-line connections.
The principal benefit of cable telephony is that it requires much less bandwidth to transmit the same amount of data as a conventional phone. Similarly to digital cable television, this compression of data moving over the cable network makes it possible for the network to carry more data at once than would be possible with older technology. This more efficient use of bandwidth allows for higher sound quality. As cable telephony uses the same network as cable internet connections and cable television, some cable companies offer these services bundled together, which can be cheaper than receiving telephone, Internet, and television services separately. This is commonly called a triple play service.
The principal downside of cable telephony is that users of the service share bandwidth with other cable telephone users and with other uses of the cable network, such as television signals and cable Internet connections. This means that, when the cable network is being heavily used, there is a risk that some users' calls will be dropped or delayed due to insufficient bandwidth to meet the demand. In addition to the possible inconveniences it can cause, this also poses a potential safety risk if someone attempts to make an emergency call, such as a call to the fire department or hospital, during peak usage hours and is unable to connect immediately.
Another disadvantage of cable telephony is that it requires an electricity source in the user's home to make or receive calls. Conventional telephones receive a supply of electrical current sent over the phone lines by the phone company, called phantom power, that allows them to send out calls even from a building that has lost electricity. This cannot be done over a cable telephone connection, though some cable telephones are sold with emergency battery backups to deal with this issue.