My extension is two four double 0.
我的分机号是2400。
The extension of the subway will take several months.
扩建地铁需用几个月时间。
He's been granted an extension of the contract for another year.
他的合同获得延期一年。
You can reach me at 637-2335, extension 354.
你可以打637-2335这个电话号码，转分机354，与我联系。
What's your extension number?
你的分机号码是多少？