Landline Phone Speaker
For the sophomore haunted house that I led we wanted to create a telephone prop that would play a recorded message, so I decided to rewire two landline phones to work.
The rewiring part was fairly simple, as I just had to open up the phone and bypass the actual phone parts so that the cable would just go straight to the speaker. The issue I ran into was that it was far too quiet, so I ended up ordering amplifiers off of amazon. After that, I just needed to acquire a bluetooth to headphone jack converter, and it worked!
Unfortunately I barely documented the process, but I have a video of one of the first times I was able to play sound through the phone speaker from my MP3 player (the blue thing):