что значит эта запись: "ACK is not received"?

[anonymous]

16:16:15.116 [CM503008]: Call(500): Call is terminated

16:16:15.116 [CM503008]: Call(500): Call is terminated

16:16:15.100 [CM503008]: Call(500): Call is terminated

16:16:15.069 [CM503020]: Call(500): ACK is not received
16:15:43.085 [CM503007]: Call(500): Device joined: sip:[email protected]:5066

16:15:43.054 [CM503007]: Call(500): Device joined: sip:[email protected]

Подскажите, пжс, что значит эта запись: "ACK is not received"? После неё обрывается звонок.

 

[anonymous]

нашёл https://www.3cx.com/forums/ack-is-not-received-19409-30.html