@gspearson, possibly when the text alert feature was set up for 1571 your landline number was entered instead of a mobile number. When the text is sent to your landline to say you have a new message it may then be converting the text to speech. Did you call the number I posted to see if you can disable it?
@NeilO Just tried that. It offered to setup a link to my mobile and asked me to set a PIN (after confirming my landline number). Clearly, as I expected, I don't have a link to a mobile nor a PIN - I would not have set up such an arrangement as I don't use my mobile for calls and it doesn't make any sounds.
I'll regard it as an unwanted benefit of the recent upgrade and ignore calls from my own landline number.
@NeilO Apologies, I've just searched my emails and found one in September from BT giving me a PIN but without saying what it was for - email was in the deleted folder.