"extension number" is a correct and accepted phrase in written English.
You can use it to refer to the number you dial to call someone's extension in an office or organization. For example, "I didn't have the correct phone number, so I called the main reception desk and asked for John's extension number."
Eventually he was connected with a lady who said that she didn't know Mr. Hooper's telephone extension number.
He must have got the extension number wrong, for the voice that answered said, in a tired manner, "War Department, Military Secrets".
By James Thurber The New Yorker, January 3 , 1931 P. 19Parody on the changing of the telephone numbers by adding an extension number to the regular number.
Incidental Intelligence: Under the heading: "Imperial Palace" in the Ethiopian Telephone Directory, which is printed in English, there is an extension number for the Chariot and Vehicle Department.
Incidental Intelligence: Under the heading: "Imperial Palace" in the Ethiopian Telephone Directory, which is printed in English, there is an extension number for the Chariot….
One former civil servant in Australia said that, in contrast to the current process, Capita "not only offered an efficient service, they were always prompt answering emails, personalised their communication with their name and extension number … The phones worked and you could get through".
On Locke's account of bodies (Book 2, Chapter 8 of his Essay), all bodies possess "real" or "primary" qualities (solidity, extension, figure, number, and motion).
A national directory of County Extension numbers is at www.reeusda.gov.
If you're calling from a number that the service doesn't recognize or from an office whose phone system uses extension numbers, you have to plug in your phone number and password to prove that you're you.
ArialPhone doesn't understand extension numbers, and things can get ugly when you connect to automated systems.
I couldn't really read yet, so I memorized twenty or so extension numbers for the most frequently requested employees and departments.
