The actual line number is also sometimes referred to as a billing number.
Say a company purchases 100 lines and they are allocated the range 0161-123-0000 to 0161-123-0099, those can be pooled together by the company for use. That grouping is allocated a "billing number" which is normally the first number in the range, but could be any of them.
When a call is made from any of these it would then normally carry the billing number as the presented number so the different networks involved know where any carrier charges need to be made to, but there is also an option for a different number to be presented, something starting 0800 or 0844 for example. When the call is made both numbers are visible to the carrier with message saying "bill call to 0161xx but present CLI as 0800xx".
Really depends on how the company is set up, smaller ones won't be very complex, but larger companies could have say 20,000 individual lines grouped into multiple bundles, in theory each of those bundles would have a different billing number, but all 20,000 present the same 0800 or 0844 number.
Interestingly though some mobile carriers completely ignore the request to present a different number, and will always present the actual/billing number. Also some customers of companies are now having issues with the network blocking due to the billing/actual number being restricted, as it means even if they have requested a callback for say a service related issue the call is being restricted in the network and never reaching Call Protect/Talk Shield system for the customer to make the decision.
Edited by dippylicious (Sat 25-Nov-17 11:34:22)