Cisco IP Phone with Passive POE

 Power Over Ethernet (POE) is a way to run devices using one ethernet cable that utilized the normal 2 pairs for data and the remaining 2 pairs for DC power. There are several standards for this including IEEE 802.3af and 802.3atwhich you can explore in more detail elsewhere as well as passive which supplies any voltage applied all the time. Passive runs the risk of damaging equipment that is not POE compatible but for simple setups which never get unplugged it is a very reliable option.

Stock image of POE wiring courtesy of Cisco.

I run a PBX server at the house and have a couple IP phones that I use when I don’t want someone to have my cell number. The number is provided by voip.ms and the server is FreePBX running in a small virtual machine. The phones I use are 10+ year old Cisco SPA525G2s. These phones are great with 5 lines, a color display, a web GUI for config, and some other features I don’t use like wifi and bluetooth. When I first got into hamshack hotline (RIP) about 5 years ago, SPA phones were what they recommended and they’ve remained inexpensive on the second hand market (AKA eBay).

Over the years I’ve used a variety of options for powering them. The phones are advertised as 802.3af POE which is ~48V active power over ethernet or 5v in a barrel jack connector. Since everything ham radio is 12V, I have adapted two of the lower-end SPA504G phones with 3A buck converters that take 12V via Anderson powerpoles and turn it into 5V. I’ve also used regular 5V wall adapters and POE injectors.


Stock image of the POE+ switch I picked up.

Last year I added a 26 port new old stock (NOS) POE+ switch to my server rack after running CAT6A ethernet through the walls to all the main rooms and access point (AP) locations. Before finding that switch, I was using a passive POE injector from Texas POE and it was just a mess of wires. My APs are Mikrotik CAP AXs and they can take 18-57V on both the POE in and the barrel jack. They are also capable of putting out passive POE up to 57V. In the bedroom, to increase the number of ports but minimize cable runs, I have one of the wall jacks connected to the downstream ethernet port of the AP in the same room.


Stock image of a Mikrotik CAP AX access point.

I recently missed a call from a contractor because I couldn’t hear the phone ringing in the kitchen so I finally decided to add a phone in the bedroom. I decided to test the phone on this downstream port. All I had to do was go into the config for the AP, open interfaces > eth2 > poe > and turn it to always on. It worked instantly. I wasn’t sure if the voltage out was high enough (I misremembered it being capped at 24V) so expected it to fail when the current demand increased. I made a test call to 311 with no issues. It actually took less time to do all that than to write this whole post.

Now I just need an extra phone to keep in the bedroom full time.


The 525 I’m currently eyeing on eBay for the bedroom.

TLDR; yes, Cisco phones work on passive POE and this can be enabled by setting Mikrotik AP POE out to Always On.

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