The landline military slang, in this case, means you need to call in through the phone or radio (instead of the landline) for a more private discussion. Typically, this means you are in serious trouble.
What does the phrase landline mean?
A landline is a telephone that transmits signals converted from audio data through physical media, such as wire or fibre optic cable, rather than through wireless transmission as is the case with mobile phones.
Is it LAN or land line?
The correct term is landline with regards to a telephone. A LAN (Local Area Network) line is a thicker ethernet cable often used for wired connection to the internet from a computer.