英文應該點講?

Please call b back as soon as you can.

Please telephone b as soon as possible. 都可以。

As soon as possible 是習慣用語，意即「盡快」。第二句是「請盡快
覆返電話俾 b」 吧！
As soon as you can雖然意思相同，但非常用語，衹作多一選擇吧！

Dear A, please call back B.

2008-06-25 11:28:30 補充：
不需要as soon as possible, 因為無要求盡快回覆!

Please call back b on phone.

please call b on phone

跟 a 說 :

Please call back Mr./Ms. b at 2111 2111 (if you know the telephone no.)!

Please get him back to me. Thanks.