A company that has many employees and many telephones may use an "extension" system. You can call this company at their main telephone number, but if you want to talk to someone specific at that company, you can use their "extension". This is a series of three or four numbers that identifies that employee specifiically. We use "extension" like this:
My number is 555-2385, and my extension is 3021.
Thank you for calling Intratel. If you know your party's extension, you may dial it at any time. Please listen for the following phone options.