I lengthily hesitated. Should I keep the Cello-Phone in her state or clean it. The fact is that patina is part of the history and soul of an object. But regarding a metal object that oxydizes, the question is a bit different: Is the patina protective or destructive? Would it have been a vintage japanese lacquered bowl, I surely wouldn't have touched it. But in the case of the Cello-Phone, there were nasty traces of corrosion.
More, I noticed that the patina was actually a coat of tar, probably a mix of smokes and grease, becoming bogged with dust and crud. So, I took the decision to clean it. With tepid and soapy water first, then with diluted metal cleaning product. I also cleaned the inside of the tube with smoking pipe cleaners (scratchy then fluffy) and got deciliters of deep black liquid before getting clear water back. Finally, I buffed the instrument with a soft cloth, and applied lemon oil all over it (and inside).
What a discovery! Until then, I thought the Cello-Phone nose saddle and Mouth shield were made of brass... and they are nickel or nickel-plated pieces! I also thought that the long tube was tin made, plated with copper... no, no, no! It is a brass tube, which was probably electroplated with nickel, since traces still appear! Big big changes!
I stopped my "restoration" there. I do not intend to re-plate the nickel, nor restore the solderings. More would be too much.
Needless to say that the playability has been greatly improved, probably thanks to the tube cleaning. The sound is still weak, dull and windy, but the bandwidth has enlarged and the attack got a bit sharper: