RCA 21230 User Manual page 13

If you have two telephone lines on a single wall jack installed in
your home, plug one end of the straight telephone line cord into
the jack marked LINE 1+2 on the back of the base. Plug the
other end into the 2-line wall jack.
If you have two separate wall jacks installed in your home, plug
one end of the telephone line cord into the jack marked LINE
1+2 on the back of the base, and plug one end of the other
telephone line cord into the jack marked LINE 2 on the back of
the base. Plug the other end of each telephone line cord into the
separate wall jacks.
NOTE: If you connect a telephone line cord from the LINE 2 jack on the
back of the base to a dual modular phone jack, you will only be able to
use line two.
NOTE: Although you can use this two-line cordless telephone with a
single phone line, you must have two lines (with separate phone
numbers) installed to use a two-line system. You may connect this
telephone to a single-line (RJ11C) wall jack, but you will only be able to
use one telephone line (either LINE 1 or LINE 2) - not both lines
simultaneously.
Connections and Set Up
13