Number spoofing is the practice of causing the telephone network to indicate to the receiver of a call that the originator of the call is a station other than the true originating station. Whilst this can sometimes be malicious it can also be an entirely valid practice. For example we may want to use a number allocated to us as a DDI, sales number etc.
In the U.K., the spoofed number is called the "presentation number". This must either be allocated to the caller, or if allocated to a third party, it is only to be used with the third party's explicit permission.
More information can be found on the Ofcom website