A nice thing about Partner is that you can also use single-pair POTS phones, modems, or other devices by just plugging them in. In that mode, you don't get the advanced features available on the digital phone, but it's easy to put up a back-room intercom-only phone in a break room, etc.
Single pair is all that's necessary for analog (KMart cheapie) phone, or FAX, or modem.
Two pairs for digital key (multi-button and display) phones.