What were you using before you moved over to BT Digital Voice?
Is the Panasonic just a single phone, or is it DECT, with a base station unit?
Then simply forget about the BT phones, and revert back to the Panasonic system. Just plug the base unit into the phone socket on the back of the Smart Hub 2.
You will also have the advantage of not losing your stored phone numbers if the Smart Hub 2 has to be reset.
In your case, there is no advantage using the BT supplied phones. For those people without an existing system, then they may be worthwhile.
If you still have the packaging and receipt for the ones you bought, send or take them back for a refund and sell the free one.
Can you relocate the Hub more centrally to see if that solves the problem? Elevate it too if possible.
You could buy a long ethernet cable just to test if it works & then consider options for a permanent arrangement if it does.
Check the T&Cs of where you bought the phones, you may be able to return them depending on how long you've had them