A backup would most likely show us that you have specified the Update Server via the Web UI:
Utilities > Software Upgrade
Simply clicking on "Clear Upgrade Server" should remove this.
Another reason may be this:
Feb 02, 2016 Question: How can I prevent a Poly Phone to be downgraded/upgraded automatically via Skype for Business and/or Office365?
Resolution: For Skype for Busines please download, unpack and import the attached LyncProvisionDeviceParams. For Skype for Business Online / O365 in addition please set the EnableDeviceUpdate to false as explained => here <=
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Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels. Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's