Is correct to say: "John, there is a Mr. Smith on the phone for you" [duplicate]

Solution 1:

The indeterminate a here implies that the speaker - and probably the listener as well - do not actually know Mr. Alan Lloyd and Mrs. Millie Preston. They're just names on the will, not actual people they are familiar with. The use of the a here means that this person could be any Alan Lloyd out there, since there isn't a concrete person to refer to.

This is the same with the phone call situation. "A phone call from Mr. John Smith" is a specific John Smith that the receiver is assumed to be familiar with. "A phone call from a Mr. John Smith" is a call from a stranger who introduced himself as John Smith.

Solution 2:

When a name is preceded by the indefinite article in that way, it usually means that the person referred to is unknown to the participants in the conversation.