Which Bluetooth sub-profile is necessary for a hands-free interface to access the phone's contact list?

The Bluetooth sub-profile necessary for a hands-free interface to access the phone's contact list is the Phone Book Access Profile (PBAP). PBAP allows the exchange of phone book objects between devices, which enables the hands-free system to access and retrieve contact information from a paired mobile phone. This capability is essential for functionalities such as dialing or displaying caller ID on hands-free devices, making it integral to the user experience in hands-free communication.

In the context of wireless audio and control, the Audio/Video Remote Control Profile (AVRCP) is primarily used for controlling media playback, while the Serial Port Profile (SPP) facilitates basic data transmission over Bluetooth, neither of which is designed for accessing a phone's contact list. The Hands-Free Profile (HFP) enables voice communication and control features with the phone, but it does not itself provide access to the phone's contact information; it works in conjunction with PBAP for that purpose. Thus, PBAP is the correct sub-profile for the requirement of accessing the contact list via a hands-free interface.