We don't have synonyms for sidetone yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are sidelong, sideline and sidebone.
(n) :
The effect of sound that is picked up by the telephone's mouthpiece and introduced (at low level) into the earpiece of the same handset, acting as feedback; or a similar effect in radiotelegraphy, public-address systems, etc.
(n) :
The effect of sound that is picked up by the telephone's mouthpiece and introduced (at low level) into the earpiece of the same handset, acting as feedback; or a similar effect in radiotelegraphy, public-address systems, etc.