extension number
Refers to the internal telephone number within an office or organization.
The extension number is 1234.
Please tell me the extension number of the person in charge.
電話番号 is a full external phone number, whereas 内線番号 is just an extension number used inside an office phone system.
Compound of 内線 (ないせん, 'extension line') and 番号 (ばんごう, 'number').