We have been struggling with an issue with handsets unregistering on 3CX V15 running on Windows 2016 Server guest OS on a Windopws 2012R2 Hyper-V host for a couple of weeks. There are 127 handsets and a smaller number of 3CX clients in CTI mode. We have previously though this was something to do with the May Cumulative Microsoft security update on the host (because that update caused other issues) but now we are not so sure - this may have been co-incidence. Unfortunately we didn't have the luxury of doing clean troubleshooting where we only changed one thing at a time, but the issue now seems to have gone away. One of the changes we made was to give the 3CX server a separate Virtual Switch which only it used - effectively giving it a dedicated NIC. Has anyone come across anything similar of does anyone have any comments that could help us shed some light on what happened?