An organization with a centralized dial plan has user extensions as the last four digits of the external DID number. Some users at the head office have overlapping extensions with the users at the branch offices. Which action fixes this issue without changing user extensions and allowing users to call each other across sites?
A. Use site codes.
B. Use different CSSs for both sites.
C. Place both extensions in different partitions.
D. Change the number of digits in the internal extensions.
