I am not endorsing Radio Shack parts in this list. Radio Shack parts are generally available world wide and they make good references for part types and prices. All Radio Shack parts listed are from the 1998 catalogue, and prices are in US dollars.
One small, portable, battery powered, amplified speaker.
If you can find a source, speakers with a metal grid on the front are the best because you will have to remove the grid to give the user access to the speaker cone. If the front is metal, it can usually be removed by bending tabs inside the case. Plastic fronts are normally molded as part of the case. The user must be able to touch the speaker cone.
Originally I thought the speaker would be the easiest part to find because I believed an amplified computer speaker could be used. But researching the project I discovered computer speakers are wired together, and use 2 batteries in each to achieve a total of 6 volts. The amplifier won't work on 3 volts. So unless you are prepared to modify the battery compartment to use 4 batteries, you need to find another source for the speaker. The speaker I used, was bought at a drug store about 6 years ago. The set of 2 was intended to amplify a pocket cassette player. Each one used 4 AA batteries. You may have to do some searching to find speakers like this now days. It may also be possible to construct the required system from a small amplifier, speaker and box. In the 1998 Radio Shack catalogue, there is a mini audio amplifier on page 175, (Part # 277-1008 $11.99) which would work, but you may have to do some cutting of the plastic case to allow the user to access the speaker.
One modular phone cable with rj11 type plug on at least one end. rj11 is the standard 2 or 4 conductor modular phone plug used in the United States.
Two LEDs (light emitting diode) any color, such as Radio Shack #276-066 or equivalent, or one bidirectional red/green LED such as Radio Shack #276-207 or equivalent. Do not use a blinking LED.
One isolation transformer. This can be either Radio Shack #273-1347 or equivalent, or the transformer from an old modem, see examples above. You must have a transformer. The project will not work without this part.
A length of small insulated wire, about 22 to 28 gage. You can cut off about a 6 inches from one end of the phone cord mentioned above. That should give you enough for the project.
One spst (single pole single throw) mini switch such as Radio Shack #275-612 or equivalent. This allows the user to turn on and off the phone line without having to unplug the unit from the jack.
One 4 pin rj11 modular phone cord coupler such as Radio Shack #279-358 or equivalent.
Two alligator clips such as Radio Shack #270-375 or equivalent.
A wire strain relief to hold phone wire where it enters the speaker case.