QSIG supports which of the following ISDN supplementary services? (Choose three.)

QSIG, which stands for Q-signaling, is a signaling protocol used to support communication between private branch exchanges (PBXs) over ISDN. One of its key features is its ability to support various supplementary services that enhance the functionality of call management within telecommunication systems.

Call transfer is indeed one of the fundamental supplementary services supported by QSIG. This service allows a call to be redirected from one destination to another, maintaining the active call connection. This functionality is crucial for efficient communication in a business environment, enabling users to effectively manage calls without needing to hang up and redial.

In addition to call transfer, QSIG also supports other services such as calling party identification, which allows the receiving party to see the number of the incoming call, enhancing the call management experience. On the other hand, voicemail retrieval and generic functions might not specifically pertain to the standard supplementary services offered by QSIG.

To summarize, call transfer is a critical feature of the QSIG protocol, providing essential functionality for effective communication within ISDN environments.