What is an ‘x’ used for in a phone number?

In a phone number, an ‘x’ is used to show a private extension.

Once the call to the phone number before the ‘x’ is dialed and the call is connected, there is often a prompt (such as “if you know the person you are calling please dial their extension”) where the part of the phone number after the ‘x’ can be dialed.

Private extensions exist because for many large organizations it is easier to have a private phone network than to have a phone number for each user in the organization.