No, you don't. You may connect two corded phones to this device. If you have a cordless phone with additional extension phones associated with it; connect the main base to the device and place the extra sets in the room(s) desired.
Additional info (see my 1st Answer):
If using home wiring, after you plug a tel. cable from the ST Home Phone box to one of your existing tel. jacks, any phone on one of those jacks in another room will be "hot" (able to make and receive calls).
So as to not "waste" one of your jacks with just the ST box plugged into it, you can also plug a wired handset into the OTHER tel. port on the box so that same jack allows you to make and receive calls where the box is plugged in, but also makes all the other jacks in your house "hot" for additional wired handsets.
All this applies to the wiring for a single line (one tel. number you had before). If you have a 2nd wired line, it prob. won't work for that line also since they are usually wired separately. However, if you're handy, you might be able to combine lines in the tel. box on your house if it doesn't add too much resistance to the wiring system... try it and see, if this applies to you.
No! It's an ingenious device.
You only need to plug the ST box into power and one wired outlet or one wireless telephone device into the ST box.
If you plug tel. cord from the box into one of your existing wired jacks in your house, that makes your existing tel. wiring system "hot (so to speak) and any other tel. handsets plugged into other house jacks can make and receive calls.
You can also use a wireless tel. system. I have an 8-handset wireless system with the base unit (answering machine and 1 handset) plugged into the ST box and that base unit communicates with the other 7 remote handsets for a whole-house solution... we even have a handset in the bathroom... comes in very handy at times!