Actually, it's not just because we're not O2 customers - it's because money will have to change hands for GiffGaff to use it. Here's a reply from Technical in the Archive:
Status: Under Consideration 19th May 2015 - Good Afternoon folks, this idea is currently on the backburner. It's something we love the concept of, but with how saturated the market is with Whatsapp Calls, Skype, Viber and others, we can't see the argument for supporting the resources it would take to create and maintain this feature. We'd need a strong case that people would use giffgaff's feature over the popular apps like Skype, Facetime and Whatsapp.
I agree that we don't need another wifi calling service. However, we do need another wifi RECEIVING service.
I live in a house with poor O2/GiffGaff coverage. It means I can't use my mobile GiggGaff number to receive calls. They all go to voice mail and sit there until I can move up the road. In other words, my GiffGaff number (and my wife's and my daughters' numbers) are useless for receiving on their numbers.
Coincidentally, I have an O2 Boostbox unit but no O2 registered business number so, again, I can't use it to receive on GiffGaff. Lots of discussion on that here in Community, too.
The technology is available (my amplifiers.com) for boosting weak signals - but it's already here in my home wifi.
Come on, GiffGaff - after another (nearly) 2 years, bring TuGo to the "front burner" please. The required "strong argument" is for better GiffGaff coverage around the UK - not to replace or compete with WhatsApp.
Agree entirely. My coverage is poor also and I have missed countless calls not able to connect to me which have I have had to pick up from Voicemail later. Normally it is from engineers who say they are on their way or not!
Ok, a quick update. I've let my GiffGaff credit go to zero.
TuGo works well, all around the house and in the garden i.e. where we have Internet/Wi-fi coverage. It automatically knows when you have lost your (3G) signal and have wifi Internet - in shopping malls, abroad, wherever. It also has good SMS and voicemail. Otherwise it uses the O2 network same as GiffGaff.
I tried BoostBox using a business contract but couldn't get the hardware to work with BT home hub - all looked good but "not registered" message even though we were. Honestly though, BoostBox has been well superseded by TuGo, which is why they used an app and your wifi rather than their hardware.
I bought an O2 monthly contract: Unlimited minutes and texts + 5Gb data (which you can top up) for £16/month for 12 months. That with TuGo means I have what I want. I liked GiffGaff but you let "poor coverage" drop, sorry.