I have just been for a quick local to test the Samsung S6.
LTE worked to about 4000ft. It worked even when the phone was showing no signal.
HSPA (showing as H or H+) worked to about 2000ft.
But there was no LTE over the SE corner of England i.e. Kent / E Sussex.
No LTE (also called 4G) at all at 5400ft.
While I don’t know the specs of ATT’s ‘4G’ service (AFAIK it’s LTE), I got continuous reception and ForeFlight Wx at 10.5k ft between L.A. and San Luis Obispo last year. Equipment used was an iPhone 4s (unlocked UK version, not bought with a contract).
I should add my S6 is unlocked, from Amazon UK, Portugese version (!!), on a basic Vodafone UK contract.
This is the first time I used LTE (4G). There is no LTE coverage anywhere I go to in the UK countryside; only 3G or 3G/HSPA. But where there is, it is very fast – similar to WIFI/ADSL i.e. about 5mbits/sec down.
Presumably there are regions of the world where the towers point upwards more.
I recall reading somewhere that the LTE network has different algorithms for tower handovers and is much less affected by the client device picking up loads of towers (as in aircraft).
I suspect you are correct Peter and should add that most of my route traversed very sparsely populated areas, so few towers (after leaving the L.A. basin, obviously).
It also depends very much on the app, i.e. how well it can cope with intermittent connectivity. WhatsApp on Android is a very good example, it does a great job at trying, pushing and polling. I can use it during all flights (FL200) and — sometimes with a delay — keep conversations going and even send the odd image.
Facebook Messenger is much worse, it senses disconnect and then goes through a lengthy reconnect procedure instead of just trying to get data through.
Gmail on Android is also very good, my emails pop up every now and then and I can work on them.
4G makes it almost a pleasure but it’s rare outside a few North European countries. In the south 3G is the only option but it’s good enough. During my trip last week (27h flying), I only needed the Thuraya phone once when the weather got hairy and GSM was poor over the Alps.
4g is available on different frequencies and therefore different range and bandwidth.
e.g. LTE 800 has a range of about 10km whereas LTE1800 or LTE2600 has a range of about 2km.
So in FL200 you can get only LTE800.
All the way down France, Samsung S6, zero connectivity on SMS, 3G 4G HSPA, you name it.
The Nokia808 would usually send an SMS, eventually. So I wonder if the S6 has a crippled GSM implementation, to drive traffic to mobile data.
I dont know how Achim does it at FL200. Maybe Vodafone UK is no good.
I had several bars of signal most of the time but nothing working.
On Tuesday I was flying VFR in marginal weather and I was entering an area of strong precipitation so I wanted to know how strong and how large. It took me about 3 minutes but then I had a full resolution radar image downloaded through my site.
The Google Nexus 6 (Motorola) is very good at getting connectivity at altitude, better than all my other phones. It’s always sporadic and not unreliable but good enough to get data from my customized weather site every now and then.
Try using WhatsApp at altitude, that works very well in my experience because it is very tolerant and keeps trying until the message is sent. For me it’s important to turn off data roaming when overflying a non EU state. It can hurt financially, I learned that the hard way in Serbia last year…
I thought Whatsapp was for sending photos of one’s private parts to teenage girls 
I will have to investigate its alternative uses…
Sounds like it can do SMS and web browsing.
I had no issues to get 4G close to Aachen in FL160. You probably get no connection in France as 4G roaming is not common.
