Hello 3CX community! We've successfully deployed our 3CX phone system but we have headache with the 365/Exchange Contacs Sync... Let me explain our problem... First of all - it's a Linux deployment and 3CX says you are supposed to change Authentification Method of the EWS Application in IIS to Basic instead of Windows: https://www.3cx.com/docs/manual/phonebook-directory/#h.kkqlf5cur2x The sync seems to work this way BUT: This affects all our clients in the company - Outlook is not working properly anymore after this change. Outlook tryes to authentificate, opens a Login Windows every few seconds (doesn't matter if Office 2010/2013/2016) and don't let you login. We have also tryed to enable both (basic and windows auth): Outlook works again - no login window. Sync isn't working anymore - it gives this error: 2018/03/20 18:42:05.268|687|0041|Excpt|Microsoft.Exchange.WebServices.Data.ServiceRequestException: The request failed. The remote server returned an error: (401) Unauthorized. ---> System.Net.WebException: The remote server returned an error: (401) Unauthorized. So, how do we solve this problem? Thanks!