Sadly, Google killed native SIP calling in Android 12. FFS!

It's relatively easy to get it set up again, although you'll need to install a separate app.

1. Sign up for a free Sipgate account. That will get you a UK phone number (+44), username, and password.
2. Install Linphone for Android (or via F-Droid)

Setting up Linphone should be straightforward. Enter your Sipgate username (a mix of letters and numbers) and password. The server is sipgate.co.uk. Finally, you must select UDP from the transport options.

And... that's it!

You can now call my SIP number from any phone +441234815259. It gets routed to Sipgate, which then notifies the app on my phone, which rings, and I can answer.

Sadly, because the native Android dialler doesn't support SIP - it's impossible to choose your SIP account when dialling. So you have to remember to open the Linphone app each time.

Why did Google remove SIP?

Because they're evil and they hate open standards which enable interoperability? Nah⁰.

Because mobile network operators are terrified of losing even the tiniest sliver of voice revenue? Nah⁰.

I suspect it was because close to zero people were using it. If you want to make a VoIP call, most people are using WhatsApp, Signal, Telegram, etc. I don't think I've ever seen someone advertising a SIP address in the wild.

It would be lovely if the big apps made even a vague attempt at interoperability. There's no technical reason why user@whatsapp.com can't place a call to user@telegram.org. But they're more interested in a "winner-takes-all" mentality rather than a collaborative effort which uplifts everyone.

Oh well.

So I've set up a Bananaphone. When people ring me, they get 30 seconds of my favourite song. Then the call is passed to me. The theory being that my friends get to enjoy a short ditty but con artists are likely to hang up before the call connects to me.

As detailed in last year's blog, I've set my Android phone up to make and receive SIP calls. I use the SipGate service which, as well as being cheap, has a nifty API.

It's a pretty simple service to set up. Once you've signed up and activated the developer package, set up a webhook:

Give it a URL which spits out this bit of XML:

<?xml version="1.0" encoding="UTF-8"?>
<Response>
   <Play>
      <Url>https://example.com/banana.wav</Url>
   </Play>
</Response>

There are a bunch of examples in a variety of language.

The WAV file has some specific format requirements. Specifically, it needs to be an 8kHz 16bit PCM WAV file.

And... that's it! It's a bit early to say whether it has eliminated spammers (or turned off legitimate callers). I might go a bit further and change this to a recording which says "This call is protected by spam guard. Press 1 to be connected." But this is a quick, easy, and fun solution.

I'm trying to give up my mobile phone contract. As part of that, I'm switching my voice calls to VoIP providers. For family and friends, that usually means WhatsApp, Skype, Signal and other consumer apps. For work, Hangouts, Zoom, and Skype.

But what about "normal" people who just want to dial a PSTN number?

And what about "abnormal" people who want to dial my VoIP directly?

These are my experiments using the OnePlus 5T running Android 10, using the native Android dialler.

Inbound PSTN to SIP - working!

In my previous blog post - Adding SIP calls to Android for free! - I described how to get set up with a free SipGate Basic account.

Now, if people ring a landline phone number, it redirects to my SIP. Which means my Android phone just starts ringing and I can answer like normal. It works even if I only have WiFi coverage. Magic!

Outbound SIP to PSTN - working!

Just works! Using SipGate I can call out from my phone - assuming I've told Android to let me pick which SIM to call from.

Or, I can go to the SipGate website and type in a phone number. My SIP line calls me and, once I'm connected, calls out to the other number.

As a bonus, their per-minute call rates are much cheaper than my mobile provider!

Sadly, SipGate don't support SIP-to-SIP calls. So I've set up another SIP account on my phone.

Inbound SIP to SIP - working!

Most SIP provider websites are terrible. The least-worst one that I found which was offering free pure-SIP accounts was IPTEL.org.

People with SIP phones can call me directly on edent@iptel.org. Woo!

Outbound SIP to SIP - broken 😞

When I try to call a SIP number using my IPTEL account, nothing happens.

Android asks which "SIM" I want to call from. I get a dialler screen, but it never connects. There's no ringing. No error tone. Nothing.

I'm on my home WiFi with no port filtering. I don't have a VPN running. I'm trying to call music@iptel.org - a SIP account which just plays music. I can call it from my desktop and mobile browser using the free jssip.net website.

But if I try the same thing using my phone SIP dialler - it never connects. I've tried calling a friend's SIP number - and it never connects. I've tried with and without and outbound proxy. Still nothing.

I can obviously make SIP calls via SipGate - but that only goes to the PSTN.

I can receive SIP calls - whether originating from the PSTN or other SIP users.

Perhaps IPTEL don't support it? I tried LinPhone - but that couldn't make SIP calls either, unless I downloaded their Android app. Even that was a bit iffy.

In my whole circle of telco geeks, I only found three people with SIP accounts. I could only make calls to one of those. The others failed to connect - but there was no useful info in their logs. Huge thanks to Ben and Matt for helping me test.

I'm done

At which point, I'm beaten. I'm going to give up trying to make SIP to SIP calls. As it happens, I only know one person who regularly uses their own SIP account. Hi Neil!

By the end of this tutorial you will be able to:

1. Receive calls to a new phone number, sent via WiFi to your Android handset.
2. Receive calls to your existing phone number, redirected to arrive over WiFi.

Get a SIP account

Sign up for a free SIPGate Basic account. Doesn't cost you anything. You can choose a UK landline number for free as well.

You will get a username, password, and config URl.

Configure your handset

Every Android phone is different. This is how I configured my OnePlus 5T.

Phone > Settings > Call Settings > SIP accounts.

Add the details from SIPgate into your phone.

Once done, activate the "Receive incoming calls" option.

Wait

Patience, grasshopper. Before you can receive calls, SIPgate need to verify you. They'll send you a letter in the post. This letter will have a confirmation code on it. Follow the instructions to activate your account.

The letter also sets your emergency services location which is passed along with any 999 / 112 calls.

Receive a call

Once your account has been activated, use another phone to call your new SIP landline. It should automatically be routed to your Android phone. The CallerID will show up as a number, followed by @sipgate.co.uk. Your dialler should be smart enough to pull the caller info out of your address book.

That's part 1 done! Anyone ringing your landline will get routed to your mobile via SIP. It works on WiFi, 3G, and 4G. Magic!

Routing your existing number

Distributing a new phone number is annoying. And people may not understand that ringing a landline number can be answered by a mobile. So, let's cheat!

This code will forward all calls people make to your mobile number, and redirect them to your SIP number.

**21*DESTINATION_NUMBER#

So, if your landline is 01632 960123, you'd dial this number into your regular phone dialler:

**21*+441632960123#

Anyone who rings your mobile, will be diverted to your SIP instantly. Your SIP will ring your mobile and you can chat normally.

Configuration

I wrote about call forwarding USSD codes back in 2010. Man... I'm old...

To check it has worked, dial *#21# - you should see a confirmation message on screen.

To temporarily disable, #21#

To re-enable, *21#

Cost

Forwarding a call comes out of your mobile plan's minutes. Fine if you have an unlimited call plan, but expensive if not.

If in doubt, check with your mobile network provider.

SIPGate don't charge you for routing your calls.

Firewalls

I didn't need to change anything on my ISP's router to get this to work. It also worked over my mobile data connection.

The WiFi at your employer / cafe / aeroplane may be configured to block WiFi calling.

What's next?

There are a number of things you could do with a SIP connection.

• Route calls based on incoming phone number. Always send your boss to voicemail.
• Custom voicemail greetings. Different messages for friends, family, work.
• Block unknown numbers. A good way to reduce spam, but a pain to update.
• Audio CAPTCHA. If a caller is unknown, ask them to solve a maths puzzle before they can be put through.
• Time of day routing. After 18:00 send all work calls to voicemail.
• Send an HTTP POST every time you receive a phone call.
• Use your imagination!

Right, I'm off to play with the Sipgate.io API to see what fun things I can build.