Thanks for trying to help, fr8ys. It's much appreciated.
On another forum (XDA) discussion covering the particular phone model (and leaving aside those comments therein about rooting, magisk and voenabler, none of which I understand) a couple of people resolved the issue by asking Vodafone to enable their account. If Talktalkmobile still uses that network, I was wondering if that might be the solution. (I'm sure this enabling refers to their SIM account, rather than broadband account.)
The only other thing I thought of was the APNs which, again, I don't understand. But perhaps these need to be changed.
In either of the above scenarios, input from TT is needed.
As for the phone, I appear now to have info in the "Phone Setting" bit (accessed via code in dialler) which is identical to that which the people who got it sorted had, after sorting it. (Specifically, that's "Wifi Calling provisioned" greyed-out in the "on" position. The writers said it showed blue...active...before they got it sorted.)
I was hoping that a Talktalk person might help, here, with account enabling or APNs, if none of the other customers have come across this.
I note what you say about mobile support not being available. How odd, to look at the top of the page and read "Mobile Support - For any questions about your Talktalkmobile service". Not everyone will read the pinned comment and will, instead, simply see the page title and current discussions taking place. Perhaps the presentation should be changed to save you and others from having to keep clarifying this.
I may need to speak to someone, although I now feel a dwindling sense of optimism. There are, as yet undiscovered, tribes in the Amazonian Rainforest who know how successful conversations with Customer Support tend to be. But one can only try.
Cheers and thanks again.
