Andy Burns wrote :
then BT (or whoever) already know the address of the line.

'whoever' depends on which way the wind might be blowing, so far as
ISP's - their service and charges. I swap those as necessary. I just
thought the 999 service ought to have instant access to correlate
number with address. On the very rare occasions when I have rung them,
or the less urgent number, they ask me for my address and often as not,
my phone number.