The how - depending on the protocol.

Signalling System No. 7 - ISDN User Part spec (found here: https://www.itu.int/rec/T-REC-Q.763-199912-I/en) allows you to specify both a calling party number (3.10) and generic number (3.26) (the UK spec adds an additional presentation number so you have 3). This will typically require the help of an operator which is 'connected' to the network on the PSTN. A real business case can be made; like a generic, non geo support numbers appearing on the persons phone instead of the geographical number of the office which called. Either a bit of social engineering or findings a less scrupulous operator is all you really need to do

SIP has FROM and P-Asserted-Identity headers which follow the same process