Ok if you are on-prem with locally provisioned phones and the reboot / reprovision command is not working you will have to choose a device to troubleshoot with. The feature worked on prior builds, but it also works on your current build and next.
I recommend factory resetting one phone (preferably a non critical one) and try to provision it from scratch:
1. Edit extension, delete the phone (and any other phone entries) and press OK
2. Edit extension again, ensure the phone is not there.
3. Factory reset the phone
4. It will appear as a bold New entry in your Phones tab which you can tick and assign to your extension.
Try this method and let us know if the phone starts to behave correctly after doing so. We will use this as an example to help troubleshoot the rest.
PS: Make sure you are using the latest version of Chrome, and don't use any browser plugins (ie. try in an incognito window) so we can rule out any browser based issues.