@Justin_UK unfortunately no I don't think that will work with the BT Digital Voice adaptors or the BT Digital Voice handsets as each adapter and BT Digital Voice handset is registered as a unique phone/communication channel.
What may work is is use use the ATA phone port on back of smart hub 2 and reinject the voice service back into your extension phone sockets with a truecall unit or use a DECT based multiple handset unit with truecall built in.
The digital voice adapters will not see the Truecall unit, as they connect directly to the home hub using DECT.
If you have an existing phone system with Truecall built in, then its easier to just plug the base unit into the phone socket in the aback of the Smart Hub 2, and forget about using the BT supplied phones.
How would I then go about having extensions? or am I then going to be limited to a single phone upstairs where teh SH2 is kept. Ideally need phone downstairs too that is connected to truecall. I'm not planning on using DECT cordless phones but Decor 2200's plugged into the Digital Voice Adapters
The FAQ deals with this issue.
https://community.bt.com/t5/Home-phone-including-Digital/Digital-Voice-FAQs/td-p/2207485
Quote "If it really thought necessary to use corded phones in existing extension wiring, this can be achieved if broadband service is via FTTP (Full Fibre to the home) by simply disconnecting the incoming wires from the master socket and connecting the green socket on the hub to any of the phone sockets with a cord consisting of a phone plug at each end"
You do not use any DV adapters.
To add on top of @Keith_Beddoe 's last post.
Truecall themselves also state "If you have phones plugged into multiple extension sockets we would recommend that you use a multi-handset cordless system"
So if you want to use truecall and have multiple handsets you may need to use a DECT based multiple handset unit with truecall built in. Then plug that into the ATA port on back of smart hub 2 and don't use the BT digital voice adaptors
And now kind of back to where I started in wanted a corded phone plugged in as these modern hand held cordless phones are not liked by all. And we need something like the old phones that just plugged into the wall. Why or why is it so complicated.
My next thinking would be extension wires running from upstairs to downstairs but even that I'm not sure would work. I will have to talk with truecall tomorrow - see what they can do.