An issue exists in 3cx with several handsets – specifically, calls do not complete when in a queuing system, leaving a cryptic error message “410 The user existed once but is not available here anymore”
3cx have the details listed on their site, no other information seems to be available.
Having troubleshooted this issue with wireshark, we noticed the INVITE being issued as the call was picked up. Turning off the “support ‘Replaces’ header” in the extension configuration of the culprit phone resolved the issue. This checkbox is checked by default on the 3cx phone system.
The above solution worked for the following handsets: Unifi UVP & Panasonic KT-UT Series.