BT, and other telecoms services offer business line users with PBX that has DDI (direct dialling in) the facility to always show their public number to caller ID devices. That means that calls initiated by an employee dialling directly out, (bypassing the switchboard) doesn't show the actual line number that they used for that call. In the case of a large installation, the PBX might have (say) 10 lines to the local exchange, (each of which has a nationally accessible number). The initiator of the call has no control over which line was allocated to their call, at best it might ring the same phone when there was nobody to answer it, or it may be barred altogether as a direct dial from outside. So the business can opt to tag the outgoing calls for caller ID purposes with a single number on which all incoming calls are received, (usually via a PBX switchboard operator, or these days more likely by navigating a menu system).
Inevitavbly, some scrotes out there have tools that they can hack the system with and get your caller ID to show a different number, e.g., living in St Albans, I would normally be less suspicious about a caller ID with a 01727 area code than an 'international' or 'withheld', - that was until there was a call from 'Microsoft Support' with somebody telling me that they had detected a virus on my PC and what he would guide me to do in order to fix it. He was wasting his time with me, but I decided to have a play and waste a lot more of it by giving him erroneous screen messages as responses to his instructions. after nearly 15 minutes he lost his temper and told me to **** off! That's what I call a result.