Which jack is primarily used to connect a digital communicator to the telephone line?

The jack primarily used to connect a digital communicator to the telephone line is the RJ31X. This specific jack is designed to allow for the connection of a security system, such as a digital communicator, to a telephone line in a way that maintains the normal function of the telephone line. The RJ31X connector enables the alarm system to dial out without interfering with other telephone services on the same line.

When the alarm system is installed with an RJ31X jack, it allows for easy disconnection of the system from the telephone line during servicing or if an alarm is activated, which is a crucial feature for alarm signals. In contrast, other jacks like the RJ11 may be used for standard phone line connections but do not provide the same level of functionality required for alarm systems. RJ45 is primarily used for Ethernet connections, and RJ12 is used for telephone lines in some multi-line setups, but neither is tailored for alarm communicators in the same way that the RJ31X is.