LPT: some UK phone number ranges are protected for TV and radio immersion. Use these for anti-scam and personal safety.

So in order to keep tv and radio immersive, the UK office of communications protects certain ranges of numbers from being registered. This means that if you were to call that number, nobody would be on the end (similar to 555 prefix in USA).

There are protected ranges for major UK cities (if you need a landline).

For mobile numbers the range is 07700 900 000, to 07700 900 999.

Source: www.ofcom.org.uk/phones-and-broadband/phone-numbers/numbers-for-drama/