Verification code issues

[Austin Prefect]

On 2 separate sites this weekend, I've been told A 6 digit code has been sent to my phone and it hasn't turned up after several minutes. On hitting 'resend' a code has instantly turned up, as I'd expect, but it hasn't worked presumably because it was the old code. But why doesn't the code come through immediately as it used to? Why does the old code come through as a resend?

[FlyVintage]

I’ve encountered much the same issue with multiple purchases over the last few days. However, after waiting for 2 or 3 minutes the initial code has eventually turned up.

[Riley Blue]

Could systems being sluggish be due to the extra volume of once-a-year payments being made for membership fees, annual subscriptions? I've made nine in the last few days.

[Paul Drawmer]

Our mobile coverage here is rubbish.
Are you reliant on wifi calling, and is it still set up on your phone?

Without it, I'm wandering around the street holding my phone in the air to try and get a code.

[Colonel Cupcake]

I get the same issue every now and again. I just turn my phone off and on again.

[Harpoon]

Worth investigating if your phone and carrier can support SMS over WiFi (which, IIRC, is part of the WiFi Calling spec'). Saved me having to go and stand in the middle of the back garden to get an authorisation code.