~ read.

SIP Mobile Phone

It has long been a possibility to use SIP services from a smartphone provided you didn't mind using a separate application and operated with the presumption you'd have good data everywhere.

However if like me, you don't believe in these things, it always seemed ideal that if there was to be a way to make a mobile phone subscription a simple SIP endpoint, having the GSM network be a simple transport layer, then these problems will be solved.

The main desire of mine was to bring a few advantages of running my own PBX to my mobile. Mainly, the ability to answer calls on any device, not just the mobile I forgot the location of, the ability to use SMS from the comfort of a real keyboard, over email, and to record calls.

Luckily, in the UK, we have a provider which allows for precisely that - Andrews and Arnold SIP2SIM, an O2 MVNO. Using their SIM we can add a GSM based extension to our PBX without any faff involving forwarding numbers or VOIP Android applications. As an extra benefit it even lets you roam onto any UK network (except Three) so you should never be without coverage. You can set this at will or automatically using a small embedded SIM application.

Benefits of the PBX Mobile Extension:

  • Record calls
  • Keep an infinite call log
  • Call transfer
  • Perform policy-routing (i.e. straight to voicemail if at certain times)
  • Fancy applications (i.e. voicemail to email, voicemail to transcription service)
  • Calls can be answered on other SIP devices like your desktop
  • Calls going to other SIP devices can have their CallerID injected by services such as OpenCNAM

Not without challenges

This is all great, however there are some very real problems, quite specific to the UK. Firstly, 07[1-9] VOIP numbers (mobiles) are not as readily available as their geographical counterparts. And you really do want an 07 number, because organisations, especially those utilising automated systems, appear to have quite a number of issues sending SMS to geographical numbers capable of receiving them. Worse, some web developers and SMS providers fail to realise that geonumbers are even SMS-capable, and will just completely prohibit you from entering it into their web forms.

Secondly, when you do use an 0744 VOIP number, some still have issues reaching you. Virgin callers for example, cannot connect or SMS you, GiffGaff users are forced into using their allowance, and other providers may have special cost considerations for your 07 range. Sometimes, this means the providers of SMS and Call services for automated systems fail to contact you and fail silently (WhatsApp number verification being a notable example). That said, the main networks are without issue. It is the smaller MVNO's which produce problems, more often than not due to routing issues internally. The 0744 range has had such problems since at least 2011. But many networks seemingly have many routing issues with MVNO's in general, never mind people being as ridiculous as me.

The telephony routes of fail

This can all be overcome by using a SIM from a regular operator in a GSM Gateway. However voice quality takes a little hit, and the extra latency can be noticeable. Not to mention most operators prohibit them in their T&Cs - although honestly if you're not sending 1001 SMS's and Calls I doubt most would care. I have one for an old Three SIM.

Late 2016 07 update

In great news, Andrews and Arnold have acquired a new range of 07 numbers within EE's number range. Meaning that when using such a number, none of the previous issues are present. Everyone can call and SMS you without issue. The downside is this is significantly more expensive than Anveo, but it does mean one can entirely do away with their GSM Gateways and even port existing UK numbers in.

Doing it anyway

If this isn't enough to deter you, as a shopping list obtain the following;

  • Andrews and Arnold SIP2SIM
  • VOIP number (I.e. An 07441 number from Anveo)
  • PBX (I use Asterisk and FreePBX]

Point the SIP2SIM extension at your PBX



  • Hostname: Where your PBX is located on the internet
  • SIP Username: Generally the name of the extension as configured in your PBX
  • SIP Password: A password for the extension being used
  • SMS URL: Where you want SMS's originating from your phone to be sent to for processing. We'll deal with this in another post.

Configure the PBX

In FreePBX. Navigate to Applications > Extensions, and Add a New Extension

Key fields which must be filled in;

  • User Extension: The same as the SIP Username from the A&A control panel
  • secret: The same as the SIP Password from the A&A control panel
  • Display name and SIP Alias can be anything you like

Everything else can be left as default or configured as desired. On saving and applying the configuration, you should be able to dial the mobile phone's SIM by dialing only its extension (from another internal extension, such as 87871 in my example). Or, if it is assigned from an internal route, by the external number.

Furthermore, you can add the new extension to your existing dial plans and routes. Adding your 07 number from Anveo works just like any other Inbound Route - http://www.anveo.com/faq.asp?code=sip_freepbx