Bluetooth Classic Hands-Free Profile (HFP) — audio + vendor AT commands.

Use this for headsets, PTT-equipped earpieces (Hytera EHW02, etc), and any device that exposes an HFP control link plus an SCO audio link.

See MobBluetooth for pairing, discovery, and disconnect — those are device-level concerns. Profile-specific operations live here.

Typical flow

# 1. Pair (only needed once per device — MobBluetooth.pair/2)
socket = MobBluetooth.pair(socket, device)
# {:bt, :paired, device}

# 2. Connect HFP profile
socket = MobBluetooth.Hfp.connect(socket, device)
# {:bt_hfp, :connected, session_id, payload}

# 3. (Optional) subscribe to vendor AT commands the headset emits.
#    Hytera EHW02 fires +CTXD on PTT press, +CUTXC on release.
socket = MobBluetooth.Hfp.subscribe_vendor_at(socket, session_id)
# {:bt_hfp, :vendor_at, session_id, %{cmd: "+CTXD", args: ""}}

# 4. (Optional) bring up the SCO audio link (audio then routes through
#    Android's normal in-call path; see "SCO audio" below).
socket = MobBluetooth.Hfp.start_sco(socket, session_id)
# {:bt_hfp, :sco_started, session_id, %{address: ...}}

# 5. Disconnect (one canonical path — MobBluetooth.disconnect/2)
MobBluetooth.disconnect(socket, session_id)

Vendor AT commands

HFP defines a small core AT vocabulary (call control, volume, ring). Headset vendors extend with their own +-prefixed commands. Subscribing via subscribe_vendor_at/2 delivers any unrecognized AT command from the headset as {:bt_hfp, :vendor_at, session_id, %{cmd, args}} for your app to interpret.

Sending a vendor AT command to the headset is send_vendor_at/4. Standard responses (OK, ERROR) are emitted by Android automatically — use the response argument to override only when the AT spec demands custom payload.

SCO audio (link control only)

SCO (Synchronous Connection-Oriented) is the real-time bidirectional voice channel HFP uses for call audio. start_sco/2 brings the link up (via startScoUsingVirtualVoiceCall + MODE_IN_COMMUNICATION) and stop_sco/2 tears it down.