The Skype for Business client merges contact data from different sources when displaying the contact card. The merged data is used to perform name resolution when performing a user search. In a more complex deployment scenario where an email domain name is in shared use in two in Active Directory forests you might run into a situation where the Skype for Business client tries to use the wrong target address.
Such a scenario would look like as follows.
Forest A is used as a dedicated infrastructure for developers. When logged on to the development network the users should be able to skype with colleagues currently connected to the default office client network.
Forest B is the default office client network.
Clients logged on to forest A connect to the Skype for Business infrastructure in forest B as external clients. Forest A is used for user account authentication while forest B is used for Skype for Business connection and authentication.
Forest A runs an Exchange Server 2013 organization.
Forest B runs an Exchange Server 2013 organization and a full Skype for Business 2015 server deployment.
Both Active Directory forests to not have trust of any sort established.
Such a configuration would result in forest A users trying to contact a Skype for Business user using the wrong address. Instead of using John.Doe@Sedna-Ltd.com the Skype for Business client of Jane Doe@Sedna-Ltd.com (logged on to forest A) would try to contact John.Doe@dev.Sedna-Ltd.com. As there is no endpoint defined for dev.Sedna-Ltd.com a connection could not be established. Therefore, no availability information is available and no call or chat connection could be established.
The Skype for Business client uses merged data pulled from the Skype for Business address and from the local (Forest A) Active Directory (GAL). This results in a connection attempt to John.Doe@dev.Sedna-Ltd.com.
Enjoy Skype for Business