This is similar to the AT&T system 2000, discussed here:
https://sundance-communications.com...cs/560521/AT_T_2052_with_two_2022_phones

Here's the link to a Staples ad with the system 2000 phones
https://news.google.com/newspapers?nid=2209&dat=19921025&id=SolKAAAAIBAJ&sjid=85MMAAAAIBAJ&pg=3555,6056511&hl=en

One set is a Master, the others are Satellite phones. The Master gets 2 lines installed via RJ-14 or (2) RJ-11's. The Satellite phones just get Line 1 via an RJ-11, and via carrier get Line 2 and Intercom