The biggest difference by far I have found is whether you are getting 4G (LTE) and not just some form of 3G (HSPA, HSPA+ etc).
If you get LTE for a few seconds, you can get a lot done in those few seconds. Telegram can download several wx images in those few seconds.
I am certain that different phones behave in different ways, too. Especially as we are talking not just “GSM” but “mobile data” which needs GSM to work to start with (you dial *99# on GSM to get mobile data).
For example at some time in the past, Apple became somewhat notorious for throwing away DHCP every 10 seconds or so. This was apparently done for a very fluid user experience for highly mobile users, but it exposed bugs in many wifi access points. Such a phone would work better in an aircraft.
My (discontinued) Nokia 808 was much better on SMS than my Samsung S6, but historically any Nokia would outperform all other phones when it comes to getting a usable GSM signal. I can demonstrate that anytime I go for a walk in the countryside where I live. It didn’t do 4G though so was almost useless in the air.
Also all modern phones are plagued with serious bugs in the telephony stack (the part of the operating system which talks to the GSM system) which is why one often ends up with a phone which doesn’t receive messages until you reboot it. IOS, Android and WP phones all have these issues. They are triggered by a gradually disappearing signal, which causes the multiple databases within the terrestrial network to get out of sync and the phone loses registration. Such bugs would also affect airborne operation.
I am certain one could make a phone which works really well, by hacking the telephony stack to make it work outside its spec. But I am not aware of any phone on which this would be easy. When Nokia briefly went open-source with Symbian in 2010, they didn’t release the telephony stack source code. This code is tightly controlled, and supposedly tightly quality-controlled, because a rogue phone could cause havoc.
My explanation for widely varying user experiences with airborne mobile data is this:
I had sporadic 3G coverage up to FL100 with my old iphone 5.
The Google Nexus 6 works extremely well in the cockpit. Tested up to FL200 in many countries. Only in Switzerland I haven’t tried it because using a sat phone is cheaper than roaming in Switzerland 
LTE makes things better by an order of magnitude. Luckily it is now pretty widespread across Europe. For about a year now I barely ever use my Thuraya phone because I get all I need from my mobile phone, this includes several trips to North Africa.
4G with antennas directed upwards would be the best data uplink for aviation.
I’ve found the coverage to be Nationally bounded. Very good signal in US and NZ, none at all in UK and Germany. Good signal in Sweden and Finland, and all down the Baltics. All tested on 3G with old iPad 2 and iPhone4 devices. I wonder if some countries restrict the signal to create an opportunity for dedicated aviation in flight. In US, Foreflight using internal GPS and 3G at 5000 to 15000 has about 50% coverage and is perfectly useable with weather radar as illustrated:
I received texts and data at FL390, 120nm from Scotland. Couldn’t reply though.
I’ve had 4G (US version) at 10K+ on an iPhone 4s running ForeFlight getting the wx overlay. Worked a treat, but sucked the battery dry in about one hour.
I can confirm that NOKIA is by an order of magnitude better than Iphone (any version). I do remember making phone calls whilst flying (NOKIA E90 Communicator connected to LightSpeed ZULU) at lower altitudes 2000 – 3000 ft. I have never managed to make a phone call via Iphone whilst flying, even at low altitudes. Occasionally I manage to send sms from my Iphone but it takes several attempts typically…
I have seen a dramatic improvement in the last week or so over the SE UK. Especially over Kent.
The 4G coverage is a lot better and at 4300 ft I had fast internet all the way from Shoreham to Manston.
I guess Vodafone have found that with 4G they can make a lot more money from everybody
I received texts and data at FL390, 120nm from Scotland. Couldn’t reply though.
The reason for this is that if a phone gets only a brief connection, it will tend to download any pending SMS messages right away, but it won’t transmit ones that you have in the Outbox because – on modern phones – the message transmission is done every X minutes only and after Y retries the phone is required to abandon it totally. The old phones, say 10 years ago, could be configured to retry every 10 seconds, for ever, which was great for getting through. My hacked-firmware Nokia 808 (the one with the great 40MP camera) was modded to retry every 3 mins (I tried and failed to find a way to make it more frequent), but current regulations (sorry, no references) prevent phones from doing anything useful like that. I think that infinite retries are banned too, even if limited to say every 3 mins.
Mobile Data is less reliable than SMS because there is much more involved to get a connection, and then the app (say Whatsapp or Telegram) needs time to notice that the connection is available, and then it will try to send/receive. OTOH, SMS delivery is very unreliable these days, relative to WA or TG.
Peter wrote:
The 4G coverage is a lot better and at 4300 ft I had fast internet all the way from Shoreham to Manston.
Ha, then you are going to believe what I’ve been saying for months, that it actually can work quite well?
On my last Germany-Egypt-Germany trip, I turned on my Thuraya phone only once and that was over the med. For the rest of the trip I was doing fine with 3G/4G. Same for my trip Germany-Tarbes-Germany the other day. There I even used Sebastian’s ADL app without an ADL device over my phone with LTE. He should charge for that 
I have always said it works well if there is 4G.
What I have also said is that in most places I have flown at any reasonable altitude (not 4300ft) there was no connectivity.
I am sure this conversation will be different again in a year or two’s time…
