Hi, sipphone <--> Asterisk PBX <--> PSTN <--> Cell Phone sipphone sets up a call to Cell Phone. When Cell Phone hangs up, it takes about 1 minute for Asterisk to show the "Hangup Zap 1/1" message, after which sipphone hangs up. During the time before Asterisk shows the Hangup message, Busy Tone can be heard at sipphone. Does anyone know why Asterisk took 1 minute to hangup ? Am I right to say that Disconnect Supervision is enabled in PSTN ? Is the Busy Tone generated by Asterisk ? If that is so, then Asterisk must have known that the line is hung up. I conducted another experiment. sipphone <--> Asterisk PBX <--> ISDN <--> PSTN <--> Cell Phone sipphone sets up a call to Cell Phone. When Cell Phone hangs up, Asterisk does not hangup at all. From the ISDN messages, it shows that Asterisk receives the Disconnect message and seem to be disconnected from ISDN. However, there isn't any Hangup message shown. There isn't any tone at sipphone when Cell Phone hangs up. Is the ISDN Hangup problem related to Disconnect Supervision ? Thank you. Regards, Kwang Mien