The primary reason Cisco Unity Connection's "Import Users" process might fail to display certain users is a missing extension in the telephone number field within Cisco UCM. Cisco Unity Connection uses this extension field to associate users with their Unity Connection mailboxes.

SSO uses Security Assertion Markup Language (SAML) to exchange authentication details between an Identity Provider (IdP) and a service provider. The identity provider authenticates user credentials and issues SAML assertions, which are pieces of security information transferred from the identity provider to the service provider for user authentication. Each assertion is an XML document that contains trusted statements about a subject including, for example, username and privileges. SAML assertions are usually digitally signed to ensure their authenticity.