On Fri, 25 Oct 2019 10:44:54 +0000 (UTC), Brian Reay wrote:

I was always under the impression that the number you send to is
the
number you would call.

From the user point of view yes.

if anything else goes on in between the customer should not be
able to
change it, its part of the sim.

All texts go via your service providers message centre, this has a
number that varies by service provider and occasionally tarrif from a
given SP. It's normally set by the SP when you register the SIM but
can be chnaged manually

I note with interest that the old 2g systems are being turned off
soon,

Where did you here 2G was being turned off?

I saw a recent article, possibly on The Register.

It is still widely used by, for example, trackers and similar simple
systems using the mobile phone network.

Aye, the networks proposing to shut down 2G (to free up the spectrum
for 4G etc) has put a cat or three amongst the pigeons. What do Smart
Meters use in the southern half(*) of the country? I know it's one of
the mobile networks but not sure which one or wich technology.

(*) The northern half uses something else operated by Arqiva.

--
Cheers
Dave.