1. May I have extension two-one-one?
可以帮我转分机211吗？

2. Can I have extension two-one-one, please?
能帮我转分机211吗？

3. May I speak to David, extension tow-one-one?
我可以找211分机的大卫吗？

4. Extension two-one-one, please.
请转211。

5. Please connect me with extension two-one-one.
请帮我转分机211。

6. Could you put me through to the personnel department, please?
请帮我接人事部好吗？

解析：
通常句子都是越长越礼貌，太简短会让人有鲁莽的感觉。