It is interesting that the cellular industry views seamless transition of calls to Wi-Fi to be a big deal and very difficult. It’s a great example of how they are viewing the world through old-fashioned glasses and are not stepping back.
You see, they view calls going into a Central Office, and with cell towers that switch calls explicitly between towers. Handing off a call from one tower to the next is a big deal and is very complex.
Celllular networks like T-Mobile are still comprised heavily of circuit switched connections, and they maintain large bundles of circuits from Point A to Point B. All of this must be managed, and that is time consuming and expensive. But that is how it has always been done.
When you start talking with a cell-phone engineer about handing off a call to a Wi-Fi access point, you instantly get a contorted-face look. They then start explaining how they have no control over hotspots, they can’t control the hotspots, and concerns about quality of service, etc etc. They have lots of excuses and not an “open mind.”
Now, consider SIP and VOIP. You have a soft phone on your Android or iPhone device. You place a SIP/VOIP call. The call goes through and packets are routed. The packets route over Wi-Fi, or over a cell tower, and there is no “circuit.” They just go into the ether and show up at the other end. Voila! You are having a conversation.
So when you originate a call on a hotspot and then leave that hotspot the router in your phone just starts sending packets via the cellular network. When you arrive at work and associate with a hotspot your data starts flowing over it and your VOIP call simply transitions again.
It seems that it is time for the cellular companies to quit thinking of themselves as phone companies. It is messing up their thinking very badly.
If they think of themselves as companies that sell data networking to fill in the gaps between hotspots, then everything works just fine. They will just implement a version of SIP/VOIP and forget about circuit switching altogether.
Since there is no longer any money in voice telecommunications for the cellular folks, perhaps it is time to quit clutching the old way.