Literally: sound – trust – non – pass through
Alternately: Communications getting cut off. While any break in expected communications can qualify, it seems to be commonly used to refer to “ghosting” – when one person simply stops responding without explanation or warning in text-based communications.
Notes: This practical yojijukugo comprises two compound words: 音信 is “correspondence” or “news”; 不通 is, as its component characters imply, a stoppage; a “not-pass-through” situation.
It is acceptable to read 音 as in for purposes of this compound.