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Mr. A : (Calling to Mr. C's office and talking with the receptionist Mr. B)..........May I talk with Mr. C?
Mr. B : Yes sure! May I know your name? OR May I know whom I speak to? OR something like that

Could you please tell me at this situation what sentence could be more colloquial?

Note : MR. B's responsibility is to ask who is calling and forward the line and the caller name to the said person, so he is asking "May I know your name" etc etc.
Comments  
May I ask who's calling please?
i rather say "may i know whose on the line?" than "may i know your name?"
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User_garyMr. A : (Calling to Mr. C's office and talking with the receptionist Mr. B) May I talk with Mr. C?
Mr. B : Yes, sure! May I know your name? May I know who I speak to?

Could you please tell me at this situation what sentence could be more colloquial?

Note : MR. B's responsibility is to ask who is calling and forward the line and the caller name to the said person, so he is asking "May I know your name" etc etc.

I hope you understand that "colloquial" refers to informal or substandard manner of conversation. Taking your note into account, I'd rather use: May I know who I speak to?/ Will you please tell me who I'm talking to? Could you please tell me who I speak to?
User_garywhat sentence could be more colloquial?
A: Is C in?
B: Who is calling, please?

CJ
rain14i I'd rather say "may i know whose who's on the line?" than "may i know your name?"

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Who's calling please? is the most natural option.

Otherwise 'May/Can I take your name please?' is also quite widely used.

The other option suggested are a bit too formal for the UK and don't sound like natural expressions- they might be used in other countries though.
Sir if we use "who" in this sentence ,it needs a verb.. But it is the object of preposition" to" here ....should'nt we use whom here.

This is Mr C

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