Final answer:
In a PBX system, voicemail requires an extension number to function properly, ensuring messages are routed to the correct recipient. Softphones, Find Me/Follow Me, and fax also rely on extension numbers but voicemail is directly associated with them for message retrieval.
Step-by-step explanation:
In a Private Branch Exchange (PBX) system, an extension number is needed for several functions to operate properly. Specifically, out of the options given, voicemail requires an extension number to function. Each user's extension is tied to their personal voicemail box, allowing messages to be appropriately routed and accessed. On the other hand, softphones are software-based phone systems that also require an extension number to make and receive calls within the PBX network. Find Me/Follow Me is a feature that allows calls to be forwarded to various numbers or devices ensuring that the intended recipient can be reached regardless of their location, but it also depends on the initial call being directed to an extension. Lastly, fax services within a PBX can be linked to an extension for routing documents to the correct recipient, even though it is not strictly a voicemail feature. The increasing need for caller ID, changes in landline usage, privacy measures, and the hectic nature of personal schedules are all factors that influence the use of such features within modern communication systems.