I have been successful when using SNMP as a method to detect phones being at home, however it does require the router (layer 3 device) device to work for you.
I will mention this guide was written with SNMP 1.x (vs the SNMP 2.x, something I need to find time to validate). I also need to verify if the ARP entries in pfsense can work vs a Cisco switch, but again have lacked time to properly validate a test.