Number unavailable when Plusnet phone

[#1]

I know that when Plusnet phone from Sheffield when they update a ticket then the number is shown as unavailable.
Is there ever a case where a ticket update is actioned from South Africa and if so do they telephone and if they do is the number routed via Sheffield and therefore shown as unavailable or direct from Durban when the number presumably will be shown as International Unavailable.
The reason for asking is that we never pick up the phone when it is shown as International Unavailable as it is either a scam or spam.

[#2]

Why is a call from Plusnet Sheffield shown as unavailable anyway?
I seem to remember that this was asked once before & the answer was 'It's a technical issue'.
It seems very odd that a modern telephone system doesn't have control of it's outbound CLI.
Maybe they should talk to Gradwell - I hear they are quite good at integrated phone systems.   

[#3]

I asked a question recently regarding International being shown but no one from PN would answer.