A game of phones: VoIP at home

August 31st, 2015


Our house has three stories, with our bedroom on the top floor. It’s not really possible to achieve a direct shout from there to the ground floor, and since my beloved likes to wander the house in his pyjamas without his mobile, using mobiles is not really an option. So, like a proper engineer, I developed a massively overkill solution: install an internal phone system!


I’ve chosen to use Mitel phones because they are readily available secondhand in good condition on eBay, and we have them at work so I’m already familiar with them. The newer models support SIP, the open VoIP protocol, and they can be powered over Ethernet, meaning you don’t need a power supply to each handset. Our current inventory consists of:

  • a 5304 – newest and smallest of the phones. Despite appearances, it doesn’t support handsfree or on-hook dialling. This one has had “hotel” firmware put onto it so that the bright blue backlight turns off when the phone’s not been used for a few minutes – so we can have it on the bedside table without disturbing our sleep. Mitel firmware is available rather haphazardly on the internet, and I found that once I’d upgraded this phone to have the backlight control,  I could only find a French-language UI for it.
  • a 5212 – this is a cut-down version of the 5220: it’s a bit smaller and the user interface is a bit more limited. It has a more subtle blue backlight.
  • a pair of Dual Mode 5220s. These (and the closely-related 5215s) are the oldest Mitel phones to support SIP, but there are two versions: if the phone does not say “Dual Mode” on the sticker on the back, it does not support SIP, only Mitel’s proprietary Minet protocol.

All the phones take 100Mbit/s Ethernet, and I supply power to them via a cheap TP-Link POE switch. They have a downstream port to provide 100Mbit/s Ethernet to another device too. Note that they don’t support gigabit: Mitel sold a “Gigabit Stand” which was effectively a PoE-extracting three-port gigabit switch that fitted onto the back of the phone, but these are relatively rarely found second-hand, and can be expensive.

There are a few gotchas with Mitel phones. The SIP firmware is a bit finicky to set up, though Voip-Info has lots of details on how to configure them. You need to switch them from Minet to SIP, and then configure the SIP settings so that the phone knows its extension number, password and the IP address of the server. The web interface is relatively straightforward though it sometimes appears to fail to update silently when run from modern browsers: I’m using an old Windows XP virtual machine to access it via IE6 (joy!). Don’t worry too much about updating the phones to the latest Mitel firmware: if the version they come with works, don’t fiddle with it! I’ve wasted several evenings upgrading mine to very little benefit.


After several false starts, I’ve had success with MiniSipServer, which is available for Windows or Linux. I’m running their Raspberry Pi edition on a spare RPi that happened to already be on the network. MSS is a relatively easy system to set up, certainly when compared with Asterisk, but is not open-source. They give away a free trial, but you can claim a free licence for personal use if you review it in a blogpost. A commercial licence is $200 for 20 users, and there are free licences available for nonprofits and OSS projects.

Because my RPi is “headless”, I’m using the web-based configuration for MSS, which seems relatively well laid out. I did have to guess the default password, which turns out to be “minisipserver”. I’ve created three local users (i.e. three extensions) one for each phone, and put them all into the same Ring Group so that they ring together when the external line rings (more on setting up the external line later).

I wanted to set up a dial plan to avoid the traditional “dial 9 for an outside line” scenario, as most people don’t expect a household phone to need a prefix to dial out. The MSS FAQ gives some hints and shows that you can disable the dial-out prefix and instead look for certain numbers as externals. I’ve gone for a very simple approach. Here in the UK, all “full” phone numbers begin with a leading 0, so it’s easy to set up a dialplan rule to route all numbers beginning with 0 to the external line. In MSS you go to Dial Plan-> Analyse Called Number and define the prefix as 0, and then set the route type to External Line. I then created similar dialplan rules for 1 and 9 (since there are lots of service-related numbers beginning with 1, and 999 is our code for the emergency services) and indeed for every number except 4. I looked at Cambridge local phone numbers and none appear to start with a 4, so I’ve set my local extensions to start with a 4.

Unfortunately, there appears to be a bug in MSS: once I’d added more than a few dialplan rules they all disappear from the web interface – but they do appear to still work…

External line interface

Our household router is an AVM Fritz!Box 7390, which is a very all-singing-all-dancing unit that includes its own VoIP server and can interface to the voice line. We used its internal VoIP server in a previous setup, but it has a few key limitations, one of which is that it can’t transfer a call from one extension to another, which quickly became irritating. However, it does work just fine as an external line interface to MSS: I just created an extension on the FritzBox and put the relevant details into the External Line settings in MSS.

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