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OK in an emergency.
Not in my experience.
112 is the correct number to call in an emergency, simply because it is becoming the worldwide standard.
In the UK it works exactly the same way as 999
The GSM standard requires handsets and networks to connect such calls even in a handset with no SIM, or a prepay with no credit, or a contract 'phone that has been blocked for e.g. not paid bill.
But AFAIK none of the UK networks respect this.
I have experienced this twice now. A neighbours daughter who has abusive parents knocked on my door one night because she needed to talk to her assigned police contact. She was very frightened because her mobile had no credit, she couldn't call the direct line, and she had tried 999 and it didn't work. I tried 112 and exactly the same. We sat in my garden and used my 'phone, and the police came 2 minutes later, very impressed.
In the other situation, my 'phone was out of coverage at my place in Norfolk, another neighbour had ODed and needed urgent help (I do seem to attract them), "emergency calls only" didn't work, so I had to knock on doors to find someone with a mobile that would connect, and we tried several.
This should be publicised, and the networks forced to comply.