What is the emergency phone linked to? the signaller? (Sorry if it's a dumb question)
It can usually dial most numbers on the generic internal railway phone network, including the railway operator, signallers (the local signaller's number is usually on the label on the inside) and the electrical control room. Obviously some phones may vary.
As this functionality suggests, these phones are often marked for emergency use even if they can also be used for secondary purposes. I have sometimes been asked to dial various control or operational management functions in urgent circumstances from such phones, or have done so on my own initiative, as part of their efforts to co-ordinate the running of trains. If I am the only person around who could conceivably need to use the phone, there is no harm in doing so. You can always hang up on a less important call anyway.
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What is inside an emergency phone cabinet is it a list of numbers?
I don't think I have ever found a phone marked "Emergency" without a list of useful numbers. When I used to do more route learning, I remember regularly looking inside the phone cases on stations to check phones actually were still in there and vaguely looked functional. Maybe I was particularly committed, but nobody ever seemed to have a problem with me doing that. Quite the contrary. I have found some Emergency phones in quite odd places and it seems natural to check they are still there.