In a Panasonic system, the digital phones use the second pair in a cable. That means that any jack should be wired as RJ-14's, exactly the wiring you need to run a 2-line cordless phone.
The first 4 ports of a TDA-50 are Super hybrid, digital and analog. If you jumper the center pins of two ports to the first and 2nd pins of a cable, you will have two analog lines for a cordless and still have the 2nd set of pins from the TDA for digital phones.
I'd be asking the Panasonic technician to set it up, but you can now use a cheaper 2-line cordless. You'll have to use codes to transfer and park calls, but it works for the short time you need it.