Read all Calendars with EWS Managed API

Hello,

I'm developing my application with EWS Managed API 2.2 https://msdn.microsoft.com/en-us/library/office/dn567668%28v=exchg.150%29.aspx.

I need to read Calendars of every user in Active Directory linked in Exchange 2013.

I Can read only calendar of logged user but not other.

oService.Credentials = New Microsoft.Exchange.WebServices.Data.WebCredentials(System.Net.CredentialCache.DefaultNetworkCredentials)

System.Net.CredentialCache.DefaultNetworkCredentials = Logged User

Thanks!