In order for two phones to be able to call each other, what must be true?

A.
They must both be in the same CSS.

B.
They must both be in the same partition.

C.
They must have a CSS that includes the partition of the other phone.

D.
They must have a CSS that includes the partition of the other phone’s line.

Explanation:
Calling Search Spaces & Partitions are basic, fundamental components of Cisco IP Telephony. Phones can call other phones (or more correctly, Directory Numbers on phones) if they are in the same Partition or if they are assigned a Calling Search Space that included a Partition that the called Directory number is in. If two phones are assigned the same CSS they cannot necessarily call one another.