It appears that the change in the certificate chain that happened over the weekend may have affected Lync Phone Edition devices
Affected devices are: Polycom CX500, CX600, CX3000; Aastra 6725/6721; and HP 4120/4140.
These devices are very old and do not have a current root certificate store.
Yealink devices, because their underlying OS is modern enough to download updated certificates, are not affected.
The Skype for Business team is investigating the problem.