If the mode was set to "TX" or "Snd" and you tried to send a fax, and the receiving fax machine did not answer, then that fax machine was either turned off, or out of paper or ink. If the mode was "Rx" or "Rcv", it means your fax machine tried to receive a fax, but none was sent. If your machine was in FAX mode and it received a regular call, then the caller probably heard the fax tone and hung up. If your machine was in ANS/FAX, TAD/FAX, TAM or 5 EXT.TAM mode, it most likely means a regular voice caller heard the answering machine and hung up without leaving a message.