The "-( 21 S 22 )-" thing is a speaker in the handset. The button is above that, labeled "VAR.". Probably the main reason why essentially the entire electronic

The "-( 21 S 22 )-" thing is a speaker in the handset. The button is above that, labeled "VAR.". Probably the main reason why essentially the entire electronics is in the handset and why it is somewhat complex is that it does not use carbon microphone, but the microphone is actually the exactly same part as the speaker, used as an dynamic microphone (with the required amplification being done by the hybrid module).