Apart from the "free" bt phones provided on migration to digital voice, does Panasonic, Siemens, or anyone else manufacture phones that replicate the bt phones functionality?
You can use other manufactured home phone sets such as Panasonic, Siemens yes but you will need to plug these into the green phone port on the back of the BT Smart Hub 2 or get a BT Digital Voice adaptor so it can connect to the BT Digital Voice service via the BT Smart Hub 2.
The BT Digital Voice handset are specially made to interface with the BT Digital Voice service and connect via the built in DECT receiver in the Smart Hub 2 and specifically made to interact with the extra BT Digital Voice extra services.