http://www.freepbx.org/news/2013-04-06/subscribe-a-blf-button-to-monitor-a-voicemail-box

This requires a newer FreePBX Distro with FreePBX 2.11 or newer which provides the required hints.

For Polycom phones, we edit the (per phone) <MAC>-directory.xml file. You can also use the (global) 000000000000-directory.xml file, but it is like a 'seed' to do the initial population of the (per phone) <MAC>-directory.xml file. The 000000000000-directory.xml is only read by the phone during initial provisioning.

cd /tftpboot
vim <MAC>-directory.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!-- $RCSfile: 000000000000-directory~.xml,v $  $Revision: 1.3 $ -->
<directory>
        <item_list>
                <item>
                        <ln></ln>
                        <fn>GenVM</fn>
                        <ct>*98321</ct>
                        <sd>3</sd>
                        <rt>3</rt>
                        <dc/>
                        <ad>0</ad>
                        <ar>0</ar>
                        <bw>1</bw>
                        <bb>0</bb>
                </item>
  </item_list>
</directory>

The label meanings: