How well does T-Mobile's (US) Simple Choice plan work when in Europe?

Now that I'm back from my trip I thought I'd add my own answer with details of the experience. All of the following is using an iPhone 4S, unlocked and using T-Mobile's Simple Choice plan (originally an AT&T locked phone).

On landing at Heathrow I turned off airplane mode and my phone immediately connected to EE. I've seen reports that this could take several minutes, but I was online before the plane reached the gate (literally, exactly the same as when flying domestically in the USA). T-Mobile sent me a couple of messages:

Data was functional but, as noted, somewhat slow. After a couple of days T-Mobile tried to upsell me on a fast data pass, which I decided against.

Partly I just didn't need the faster data, and partly I had signed up for The Cloud, which offers free Wifi access at numerous UK locations. Often when I'd pull out my phone I'd discover that I was actually using Wifi data instead of phone network data.

I stayed the UK, mostly in London with side trips to Cardiff and Milton Keynes. Most of the time I was on EE, though I also saw O2, Vodafone, and Three at times. As advertised, my phone just worked. Calls and texts to my US phone number showed up as fast as usual, and calls to the US went just fine. I did not make any calls to UK phone numbers-- I expect I would have had to include the UK country code, but otherwise I'd expect them to just work.

Usually that was fine, with a couple of caveats:

• EE's coverage was not always as good as I would have hoped. Usually it claimed "3G" and worked fine:

• Nevertheless, Edge was common and GPRS showed up at times.

• O2 never worked, not at all. If I saw O2 at the top of the screen I just gave up.
• In an attempt to reduce problems with O2 I tried turning off automatic carrier selection (on an iPhone, this is Settings --> Carrier --> Automatic) so that I'd only use EE. That stopped O2 from showing up but also meant I got dead spots. Dead spots on trains in relatively remote areas isn't a surprise, but I also got them in central London a couple of times.

My verdict: Taking Simple Choice to the UK works as advertised. In some cases local carrier coverage may be less than you'd hope, but that would apply to any phone and plan.

Update: The 20 cents/minute charge for phone calls may not show up until the next month's bill after traveling. The charge applies, but it may be delayed. VOIP options like Skype or FaceTime would of course count only as data, assuming the connection is good enough to use them.

Update #2: On a later trip that covered Iceland and the Netherlands, I had similar results though without anything like the O2 problems I had in the UK. On that trip I ended up buying a T-Mobile data pass to get faster data, but the basic service worked as advertised at all times.

T-Mobile roams onto other T-Mobile networks where they exist, e.g. in Germany (T-Mobile is a German company).

T-Mobile owns 50% of EE in the UK, and all T-Mobile customers (from any country) roam onto EE when in the UK.

EE in the UK is a major network with coverage nearly anywhere you're likely to go. The only places you're likely to struggle for a signal are in rural areas (especially hilly ones), which might include being on long journeys.

I used this plan on my Nexus 4 in Israel, Germany, and Greece this summer. I used it mainly for data, with some texting and voice calls (though at 20 cents/minute, I kept voice calls to a minimum).

It was not perfect. Sometimes I had to manually scan for networks and get it to register on a different network (because automatic registration failed). Sometimes it would appear to be connected to a network but did not actually have Internet connectivity. However, overall I was happy with the coverage.

Speed-wise the data was not fast, but was perfectly adequate for email, browsing, maps, IM. But it did not allow me to download files (e.g. PDFs) from my phone browser. Trying to download files would take me to a T-Mobile My Account page. This was very inconvenient when I was trying to get PDF maps, restaurant menus, etc. (It did allow me to retrieve email attachments.)

Just got back from a month in England, France, Belgium, Netherlands. Texting worked perfectly; phone was fine at 20 cents/minute (though when wifi is available, Google voice is 1 cent per minute); the data at "2G" made me long for the old days with my dial-up modem (56k): typical speeds I got on the free international data roaming plan were 10 to 15k (not Mbps, but kbps). In other words, worthless for browsing web, intermittent for email. After about 20 calls with TMo tech support, they kindly gave me a complimentary high-speed data package upgrade. This fixed the problem, for the most part. In brief: don't believe the hype about data, but the calling and texting is great and free or cheap.

My wife and I spend two weeks in Rome, Sicily, and Malta in April 2018. Our experiences were different. She has a Nokia 950 Windows 10 Mobile and I have an Alcatel Revvl 5049W Android 7. She had good coverage with consistent 2G coverage and had no trouble sending and receiving text messages. I had less consistent 2G coverage and could receive but not send text messages. Twice I tried to make a call but only one call connected. The difference was the radio bands. Her phone was a premium phone with a lot of radio bands while mine was a $150 bargain. When I went to the settings screen showing available networks, her list was different than mine. Since Wi-Fi calling worked great for me, I called T-Mobile and asked. They told me which carriers I should see but I did not have them in my list. My wife had them in her list.

Bottom line: if your phone has the radio bands, your texting and 2G coverage in Europe will be good enough that you should not need to purchase anything else.

Used it in Fiji (bad), New Zealand (mixed, okay in urban areas not a lot of coverage in the outback), Australia (very good in Sydney), Singapore (okay) Thailand (very good in Bangkok), Germany (mixed, depending on where you are).