Who's speaking? 哪位？
Who's that speaking,please?
May I ask who's calling?
Who is that?
B:Hello,This is John.I want to speak to Linda.
A:This is her.
B:Hi,Linda.I'm just calling to invite you to a dinner party tomorrow evening.
A：Hello,This is Mary,who's speaking?
B:Hi,Mary.This is Jack.May I speak to Mr Gates?
A:I'm sorry.He's not here right now.Can I take a message?