I would suggest role play too. DS1 is going to be 7yrs in September and I have taught him from a young age to answer the phone with "Hello this .... speaking", and he can ask who is calling and so on and hold a conversation very well with whoever is on the other end. He can also make a call to his cousin (who is the same age) and hang up after they have sorted out their plans and not told the parents Lol.
I have just started with DS2 and he will be 4yrs in a couple of weeks. He answered the phone for the first time last week and got it right with "Hello this is ... speaking" just a bit slow on wanting to hand the phone over.