I tried to top up my phone earlier today (completely out of credit) and went through the usual motions.
On attempting to pay, the interface tried redirecting me to my banks security check page.
This was not whitelisted by my provider and so I could not access the verify.monzo.com web page. Preventing me from topping up…
I could not contact the help desk or get the sms fallback or call 
etc.
Thankfully, WiFi was a short walk away.
I realize this is, at least partially, the responsibility of my provider.
That being said, rolling out to everyone before the edge-cases have become visible is a little bold.
On the bright side - when I did get WiFi, it worked perfectly 