The Japanese sentences here is more like "I'm not sure of the name..." without saying anything about the extension being 211.

But doesn't 拡張子 usually mean extension as in a filename extension or an expansion slot? I've never seen it used as a telephone extension number (remember them?)
I think a better approach to resolving the mismatch would have been to add a revised English sentence, e.g. "I'm not sure of the name."

@JimBreen
Thank you for your advice. I've returned this sentence back to the original.

Thanks. I've added the matching English.

I've unlinked.

I'm not sure of the name, but the extension is 211.

➜ #322935
unlinked by CK, a minute ago

extension for phones is called 内線

So for the now-unlinked English we could add "名前はちょっと分からないんですが、内線番号は211です。"
There's already a sentence using 内線番号: (#174194) "交換手です。内線番号をどうぞ。"

内線221番 would be a possibility, too, I think.

Thank you for your detailed explanations to me, both of you. I'll add a translation "名前はちょっと分からないんですが、内線211番です。" for [#322935].

By the way, I'm troubling. I'm not sure the sentence [#174194] is OK, or Not. Sorry...

Is it the "内線番号をどうぞ"? It feels stilted to me - maybe it's just very old-fashioned.

「交換手」の方が出ている動画を探してみたのですが、音声付きの動画が見当たりませんでした。
https://www.youtube.com/watch?v=WA6E7mMYEtY

恐らく「そんな感じなんだろうな？」とは思うのですが、確信が持てません。