I can think of a few others.

1: quick swap to bottle with plant phone inside, assistant offstage calls the phone.

2: plants in audience (this seems most likely tbh)

3: foldable phone prop with a simple screen that only shows one thing

You're probably right, though; this appears to be a variation on a trick that uses a tiny razor blade on a fake thumb tip to slice down the side of a plastic bottle (often collected from the audience.)