Hi, I am curious to know whether anyone else can replicate my issue, or can suggest an avenue of investigation. I have tried it with a number of Aastra phones, and they all seem to do it (all SIP firmware 1.4.0). If the SIP call takes the form: INVITE -> TRYING -> BUSY HERE then everything seems to work fine. The phone makes the right busy noises. On the other hand, if the SIP call takes the form: INVITE -> TRYING -> PROGRESSING -> BUSY HERE then I get a very brief "blip" noise from the phone, and then silence instead of a busy tone. The first case is caused by having no Zap channels available. The second case is caused by having a channel available, but where the remote phone is actually busy. Any clues would be most welcome. I cannot downgrade to SIP 1.3.x as that has a bug with unattended transfers sometimes hanging up callers :-( Kind regards, Steve