内ない線せん番ばん号ごう

Meanings

1

noun

extension number

Refers to the internal telephone number within an office or organization.

nickname, number, item

内ない線せん
内線 means 'extension (line)' more generally, while 内線番号 specifically refers to the number assigned to that extension.
電でん話わ番ばん号ごう
電話番号 is a general telephone number, whereas 内線番号 is specifically an internal extension number within an organization.

Compound of 内線 (ないせん, 'extension line') and 番号 (ばんごう, 'number').