Game Design
The interactive nature of this piece allowed me to look at it through the lens of a Game Designer.
An audience member sitting at the table with the phone, without their knowledge, is engaging in play.
As such it is important to allow them to discover aspects of the piece and to use it as they see fit.
Therefore you have a duty to give them an opportunity to discover.
There are 5 visible buttons on the phone, it would be disappointing to have only 2 of those buttons do anything. So you must have something happen on buttons 3 through 5.
There is also a state for when the person hangs up the phone, and a 10 second grace period for them to pick it back up again before the work restarts.