Panasonic's answer about the KX-TGFA71 and KX-TGFA72 leaves out an important detail:
THESE HANDSETS ARE NOT AVAILABLE FOR SALE!!!!
Not from Best Buy, not from Amazon, Not Even From Panasonic Itself!! They are not even listed on Panasonic's site!! If you communicate with Panasonic, the answer is that "I'm sure they'll be made available 'soon' ". The system came out last year, but the expansion handsets are still not available as of 1/15/2020. So far th only way I've seen them for purchase is some folks are buying whole systems, deregistering the individual handsets and then selling them individually at exorbitant prices. This problem exists on a number of Panasonic's otherwise excellent systems, and Panasonic seems not to be interested in fixing it.
The lesson is make sure the system you're looking at already comes with all the handsets you need.