Similar story, my 14 month old mocks everything we do. If I knock on a door, he knocks on the door, if I sneeze, he'll try to make a sneeze sound, if I fold laundry, well...he'll unfold it and scatter it on the floor, but you get the idea. But since we use the phone, he wants to use the phone, there's no avoiding it.
I don't see the big issue, it's something in his environment that he's curious about. It's not a direct physical danger, I let him explore it.
Our 17 month old is similarly into imitation. Kids have fantastic imaginations. Our 17 month old couldn't care whether it is a phone or an old cardboard box that he is holding up to his ear. The advantage of a cardboard box is that it seamlessly transforms into a car or a ball on demand!
I don't see the big issue, it's something in his environment that he's curious about. It's not a direct physical danger, I let him explore it.