What does 内線 mean?
内線 is a Japanese noun meaning “phone extension” in English. It is written as 内線 in kanji, ないせん in hiragana, and naisen in romaji. This word is part of JLPT N2 vocabulary.
Example sentence
内線２１４に出る人なら誰でも構いません。
I'll speak to anyone at extension 214.