Mobile Network Access "forbidden"

[#1]

Why does this happen? ...

There have been a number of similar posts on this topic, but for the last couple of days my phone has not connected to any network.

I discovered that by un-setting "autoselect" in my SIM settings I could see all the available networks (including PlusNet) flagged as "forbidden"

I was able to click on PlusNet, which got my connection active again ... But why has this happened?

Some Googling suggests it's down to the "FPLMN list (aka forbidden network list) which is stored on a SIM and tells a device which networks it can't connect to.

But I don't know why this should suddenly block all networks, including PlusNet

[#2]

Hey there @geoffers! There's no real reason for it happening, it can sometimes just be down to the sim or the phone, but from my experience no rhyme or rhythm. 

[#3]

I wonder if the issue is actually a problem with EE (since Plusnet runs on the back of EE)

My mate has a direct EE account/SIM and has experienced a similar issue with them for the past week or more. He managed to fix this by selecting the EE network manually as the AutoSelection was forbidden 

[#4]

Happened again today ☹️
It least I know how to fix it, but would like to know what's causing it!
Suspicious that the phone had restarted overnight (software update maybe?) so maybe it's phone not network related