I have noticed some calls that do not go through and the caller on the other end hears nothing. I have been doing SIP tracing, and I found out the following according to the SIP tracing:
- For calls that fail with a 580 Precondition Failed, either the UC520 does not send the 180 Ringing response after receiving the INVITE message or the SIP packets containing the 180 Ringing response are dropped somewhere
- The 200 OK response is also not sent when calls fail with a 580 Precondition Failed message
- For successful calls, both the 180 Ringing and 200 OK responses are sent after receiving the INVITE message
- In both scenarios, the UC520 does actually send the ACK response
I am unsure whether the UC520 is sending the wrong response, if the problem is actually with the ITSP, or if SIP packets are getting dropped in between the UC520 and the SIP proxy. This problem seems to occur with the 7.0.x and 7.1.x releases.
Has anyone else experienced this problem on the UC520? If so, do you know any solutions to this problem?