noun
call-not-accepted signal
A telecommunication signal indicating that an incoming call cannot be accepted. Domain-specific computing/telecom term.
着呼受付不可信号が返ってきたため、接続できなかった。
The connection could not be established because a call-not-accepted signal was returned.
Compound of 着呼 (incoming call), 受付 (acceptance), 不可 (not possible), and 信号 (signal). The exact historical derivation is uncertain; the term is a technical coinage.