I believe the best way to clean Bakelite and plastic Telephones is as follows:

Carefully dismantle the Bakelite telephones as much as possible. Then

Clean the Bakelite with a good non abrasive chrome cleaner such as autochrome.

Working the cleaner into the telephone in a circular motion.

The next stage and most important stage is to use "paste polishing number 5" from greygate chemical products specifically designed for cleaning Bakelite telephones.

Finally finish with a good quality "carnauda" car wax (that containing no silicones)

At each stage use plenty of elbow grease and patience. For the final polish use a clean cotton duster.

A few people have asked me how to relace the old carbon granule microphones/transmitters on old 200/300 series bakelite telephones with the more modern 21a type, so I have included instructions below to do this.

Step 1) Remove the base plate of the telephone

Step 2) Inside the telephone check that the 3 wire cord is correctly installed (white from T4, Red from T5 and green on T6)

Step 3) Remove the mouthpiece cover by carefully inserting a thin prong into the hole and carefully twisting the mouthpiece anticlockwise.

Step 4) Lift out the old carbon granule microphone/transmitter.

Step 5) Unscrew and remove the spring contacts.

Step 6) Connect the new 21a microphone (transmitter) to the new cord Red and the White wires.

Step 7) Connect the Green wire as original then place a strap wire link to the red wire on the new transmitter to the original screw that held the red wire.

Step 8) Use some form of soft packing behind the new (smaller) microphone to prevent rattling.

Step 9) Replace the mouthpiece gently (as this can be easily damaged)

Good luck