Yes that is an option, however as seen by this partlist of idems needed to build a caller it is close to what the above cheap callers cost.
Radio Shack items parts list
Radio Shack #277-1008, Mini Audio Amplifier/Speaker = $12.99ea
Radio Shack #275-1565, SPST Soft Feel Push on Push Off Switch = $2.59ea
Radio Shack #276-084, LED Indicator Light for Amp (fits 3/16" hole) = $1.99ea
Radio Shack #42-2387, 6 Foot Patch Cord 1/8" Stereo Plug on each end = $4.99ea
Radio Shack #270-325, 9V Snap Type Battery Connectors (Pkg. of 5) = $1.99ea
Radio Shack #270-1805, Project Enclosure ((6”x3’X2”) = $3.79ea
The speaker found to produce the absolute BEST sound is the Speco SPC-5 PA Speaker. These can be order online from Lashen Electronics, they are $8.42 plus shipping, but they have a minimum order of $25.00. This speaker comes with about 10 feet of speaker wire and a plug attached (or at least the ones I have used have) to plug the speaker directly into the Speaker Jack on the Radio Shack Mini Amp.
For a total of $36.76, if you add in the cost of even a cheap MP3 player and add your states sales tax it is well above the cost of the above E Callers.
That is why I am looking for owner comments.
Thank you for the thought.
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Al