You have an Exchange Server 2013 Organization.
All users are enabled for Unified Messaging (UM) and access their voice mail by using Microsoft Lync 2010.
Callers report that when they press a key to Dial an external telephone number,
they hear a message indicating that the intended destination cannot be reached.
You need to ensure that the callers can reach the external number successfully.
What should you modify?
Answer: 
Explanation: