I’m not exactly clear which cable you’re talking about.
Are you referring to the cable coming into your office from the phone company (which carries the incoming lines), or the lines that run from your central phone box (usually called a Key Switch Unit, or KSU for short) to the various phones in your office?
If it’s the incoming line, and it’s about as big around as your finger or thumb, there’s a good chance it’s a 50 pair cable. They can carry, not surprisingly, 50 phone lines.
This may sound dumb, but if you can see the cable and lay hands on it, you can probably strip a small bit of the outer sheath and just count the pairs. Discounting unusual stuff like multiplexors or other line sharing devices, you can have one incoming line per pair of wires.
Each pair is colored coded, and the two wires are twisted together. Usually one of the two wires is a solid color, and the other is that same color with a stripe of white or black.
Typically, with an in-house phone system, the KSU is the center point. From the 50 pair cable (or whatever you have coming in) all incoming pairs run to “input” side of the KSU (in your case, this would be 12 pairs), and then an individual line runs from the KSU to each wall jack where a handset is plugged in.
The KSU takes care of connecting you to an open outside line when you pick up a handset, directs incoming calls from all of the incoming lines to various handsets depending on how it’s programmed, and may handle voice mail (more on this later).
The wires running from the KSU to the wall jacks may be 2 or 4 pair, depending on the phone system and who wired it. Most systems require only 1 or 2 pair for an individual handset, so you may have unused pairs running from the KSU to the wall jacks. But to use them, you’d have to put in splitters at both ends, or pull the jacks and play with wiring, etc. Don’t know how handy you may be doing that.
If you do have a 50 pair cable coming into the building (which used to be the most common for a commercial installation), you should have plenty of open pairs to bring in a few more lines.
Assuming that this is true, and that there are phone jacks where they are needed in the portion of the office you are renting out, and that those jacks have cables that run back to a wiring closet or panel somewhere (probably where your existing phone system is installed), then things are pretty simple.
Your (or your tenant) should be able to have the phone company activate the incoming lines they need, which will come in on unused pairs in the 50 pair cable.
Then you (or your tenant) gets a second phone system (a KSU that can handle the number of lines and handsets needed, and the handsets themselves) and install the KSU in the wiring closet next to the existing system.
Run the pairs that carry the new incoming lines to the new KSU, and plug the lines coming from the appropriate wall jacks into the new KSU.
Presto, you’ve got second phone system completely separate from the existing one.
Alternately, your existing KSU may be programmable to direct calls from certain incoming lines only to certain phones. In this case, you wouldn’t need to get a second KSU. You just program the existing one to send calls on the new incoming lines only to the handsets the renters are using, and Bob’s your uncle. Outgoing calls may or may not work the same way, again it depends on how programmable your KSU is.
For voice mail, they are typically just digital voice recorders with some smarts built in. They have two main functions: answer calls and put people through the “Press 1 to talk to sales” menu system, and to record messages when someone doesn’t answer their phone. They may be part of the KSU, or a separate box that can work with the KSU. They’re basically just multi-line answering machines with some extra smarts.
For the menu systems, all incoming calls are directed to voice mail system first. It answers, plays the recorded message, then waits for you to make a selection by pressing buttons or saying simple words and numbers. Modern electronics make decoding your button press or simple commands fairly easy. You then get transferred to another menu, a recorded message, or some live body’s handset. Repeat until you get where you want to be or hang up in frustration.
When an incoming call is switched to ring at a particular handset, and that handset doesn’t get answered after the appropriate number of rings, the voice mail controller grabs the incoming call, answers it, plays the outgoing message, and records the incoming message.
There are tons of options, but that the gist of it.
Lastly, if you call one of the zillion phone systems suppliers, they will probably stampede down to your office to give you a bid on a second KSU and handsets. Tell them you’re not sure if you have enough cabling in place to handle the new lines, and they’ll probably check it out for you for free as part of the process of giving you the bid on a second system.
Good luck!