Same issue here with the same phone. Was having another issue lately
with not receiving otp's that seems to have sorted itself but still
receiving 20 odd copies of SMS messages a day. I'm just glad nearly
everyone uses WhatsApp now or I'd be drowning...
No idea it's certainly frustrating. I've bought a new tablet and
struggling to install a lot of apps on it because I can't get an OTP to
verify my account details. Some of them have let me change to an
authenticator app but not all. Just as an update...
I've been having the same issue from several different companies that I
never had when I was with O2 previously. Returned to them recently and
it feels like at least half of the otp's I request never arrive. Same
phone and number I had with O2 previo...