Introduction: recently encountered a need to write a APP on-board system, you need to control your calls on your phone. Previously had contact with Bluetooth, so a little bit to understand.
first car phone is connected, the phone belonging to the service side, on-board system belongs to the client. In the Android source code to achieve the client to the server Bluetooth phone hang up and answer. Code: /frameworks/base/core/java/android/bluetooth/BluetoothHeadsetClient.java
There is a corresponding function to answer the call and hang up.

answer the phone

    /**
     * Accepts a call
     *
     * @param device remote device
     * @param flag action policy while accepting a call. Possible values {@link #CALL_ACCEPT_NONE},
     * {@link #CALL_ACCEPT_HOLD}, {@link #CALL_ACCEPT_TERMINATE}
     * @return <code>true</code> if command has been issued successfully; <code>false</code>
     * otherwise; upon completion HFP sends {@link #ACTION_CALL_CHANGED} intent.
     */
    public boolean acceptCall(BluetoothDevice device, int flag) {
        if (DBG) log("acceptCall()");
        final IBluetoothHeadsetClient service = mService;
        if (service != null && isEnabled() && isValidDevice(device)) {
            try {
                return service.acceptCall(device, flag);
            } catch (RemoteException e) {
                Log.e(TAG, Log.getStackTraceString(new Throwable()));
            }
        }
        if (service == null) Log.w(TAG, "Proxy not attached to service");
        return false;
    }

hang up the phone

    /**
     * Rejects a call.
     *
     * @param device remote device
     * @return <code>true</code> if command has been issued successfully; <code>false</code>
     * otherwise; upon completion HFP sends {@link #ACTION_CALL_CHANGED} intent.
     *
     * <p>Feature required for successful execution is being reported by: {@link
     * #EXTRA_AG_FEATURE_REJECT_CALL}. This method invocation will fail silently when feature is not
     * supported.</p>
     */
    public boolean rejectCall(BluetoothDevice device) {
        if (DBG) log("rejectCall()");
        final IBluetoothHeadsetClient service = mService;
        if (service != null && isEnabled() && isValidDevice(device)) {
            try {
                return service.rejectCall(device);
            } catch (RemoteException e) {
                Log.e(TAG, Log.getStackTraceString(new Throwable()));
            }
        }
        if (service == null) Log.w(TAG, "Proxy not attached to service");
        return false;
    }

In addition to call state management in: /frameworks/base/core/java/android/bluetooth/BluetoothHeadsetClientCall.java
here acquisition device is connected, it has been a call.

Get the current call status

    /**
     * Gets call's current state.
     *
     * @return state of this particular phone call.
     */
    public int getState() {
        return mState;
    }

    /**
     * Gets call's device.
     *
     * @return call device.
     */
    public BluetoothDevice getDevice() {
        return mDevice;
    }

At first I was developing on Android studio and found no api and maybe a jar package, maybe later found hidden classes are Android api, application systems can only call party applications not take. If you want to use the reflection mechanism can be used to invoke.