Here is a simple, no prep game that you can play to reinforce the phone dialogue. I played it today and my kids got a kick out of it:
Choose one student to come to the front of the class. This student is the "caller" and must face the board, away from the rest of the class. The caller/class follow the basic dialogue which I wrote on the board for them. For the line "May I speak to _______?" The caller must pick someone from the class. After they finish the dialogue, random students from the class should take turns disguising their voices and saying "Hello?" The caller must decided if the person is actually who they asked for. If they think the person is who they asked for, they reply with "Hello, ____." If they don't think it is the person they asked for, they should reply with "May I speak to _____?" again. This continues with different students disguising their voices until the caller guesses correctly. Then, choose a new caller and repeat.
You could also try doing this activity in groups, but I would make sure to do it together as class first to make sure they all understand. (I initially tried the activity in groups and ended up with a class of confused children.)
Sample dialgue:
Class: "Hello?"
Caller: "May I speak to (choose 1 student from class), please?"
Class: "Sure. Who is calling?"
Caller: "This is (caller's name)."
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Randomly select students to come up and say: Hello? (with disguised voice)
If caller thinks "Hello" is from the person they asked for they reply: "Hello, _____."
If caller thinks "Hello" is from someone other than the person they asked for: "May I speak to (person they originally asked for)?"
I hope that makes sense!! Good luck!!