From time to time the support team may ask you to clear your local cache to help resolve a particular issue.
This can be achieved by performing the following steps:
1. First Sign out of Skype for Business.
2. On the Login Screen click, "Delete my sign-in info".
3. Click on the arrow to the right of the cog in the top right and select File > Exit.
4. Open windows explorer and browse to the folder:
(depending on the usage of Skype for Business 15 or 16)
5. You should see a folder with your sip address for example firstname.lastname@example.org
6. Delete this folder.
7. Restart the PC.
8. Log back into Skype for Business.