This is absolutely how IEDs are made. You buy an old Nokia phone and wire the speaker/ringer to the detonator. Call phone -> boom.

In fact, TFReadme directly references Electroboom/Mehdi Sadaghdar who did this with an old-school analog alarm clock (the kind with an actual bell) and in the video he muses that it's literally a time bomb, har har get it.

Doesn't look improvised to me.