I have an undocumented green phone socket on my BT Smart Hub 3. Can I use this for a landline phone and how many phones could I connect?
Yes & no.
Yes, it can be used with a landline phone but only if you have Digital Voice (DV) with BT. Not sure how many phones it would support but modern phones don't need the amount of current to ring that old phones did, so the idea of REN doesn't really apply anymore.
Yes, it is for the BT Digital Voice service. You can connect a number of phones, the exact number will depend on the phones.
By "wired system" do you mean the extensions in the house?
If so, then basically all you need to do is disconnect the extension wires to the master socket.* Get a lead with a BT phone plug to BT phone plug and use that to connect the nearest extension socket to the Smart Hub.
*If you are still on part fibre (FTTC/VDSL), you still need the incoming phone line, so don’t disconnect that from the master socket. Your internal extensions are your property, however.
Thanks liquorice, that's what I thought but the BT man denied it. And it does indeed work with an old phone directly plugged in. But I wonder if I can connect to 3 other phones via the existing phone wiring?
Shouldn't be a problem, try it and see.