Receiving calls from unavailable numbers, why are they going to voice mail?

[iMore Question]

Receiving calls from unavailable numbers

Last night my bank tried to call me from their unavailable number and it didn't ring, it did however manage to leave a message via 121 why is this please?

 

[Bigeric23]

Re: Receiving calls from unavailable numbers

I don't have this issue. Have you noticed this problem from any other numbers?