If a SIP address can't be found, then Skype for Business will use the UPN as the SIP address. Using the credentials provided, we will try to discover the SIP address.

If the validation fails, you may need to provide the domain and user name. The UPN and password are used to validate the account in AD or Azure AD. When not checked, all Exchange services will be disabled, and the Exchange server does not need to be provided. When checked, all Exchange services will be enabled (for example, calendar on the welcome screen, emailing whiteboards). The device account’s username and password must be able to authenticate to the specified Exchange server. This is the Exchange server of the device account. Session Initiation Protocol (SIP) address This field does not need to be provided for Office 365 accounts. The domain that the device account belongs to. The corresponding password of the device account. The user principal name (UPN) of the device account. You can change the device account in Settings to either add an account if one was not already provisioned, or to change any properties of an account that was already provisioned.

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