In a phone number, an ‘x’ is used to show a private extension.
Dial the phone number before the ‘x’ first.
Next, wait for the call to be 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’ is 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.