You can use the following link to open a support case.
In your case notes please include: Parent Case 05018268 / INC-18089
Please also include your ISP as well as the SIP proxy. You can locate the SIP proxy in the desk phone's GUI, or on the Sofpthone. On the Softphone, just hit Ctrl+F1 and select "SIP proxy settings" like shown below.
This issue may only be affecting 3rd party phones, but we'll update this thread as we have more information.