An engineer is supporting an existing Cisco Collaboration deployment that has internal and external home...

An engineer is supporting an existing Cisco Collaboration deployment that has internal and external home users using the solution without VPN. Business usage also includes B2B calling for voice and video. Suddenly the engineer receives a report that one of the home office users cannot use the Cisco Jabber client, and shortly after, a few more reports come in for the same error. What must the engineer check first to resolve this issue?