Hey, magenta1782376wp. This is such a bummer to pass along, but after reading your post I did a double check on existing issues with the iOS app, and it looks like right now there is an issue getting DIGITS operating with an international SIM on iOS. Our support teams are already actively working on a fix, so although we don't have a timeline to share, I do want to let you know that 1. it's not you, it's us, and 2. we are on it. Hopefully, given the number of customers we have who travel regularly and the number of customers we have who breathe everything Apple, Venn's common area there is on the large side and this is a priority to get working again.
One month later, I'm still unable to use DIGITS abroad on iOS devices. Any clue when this issue may be solved?
I managed to work around this by cheating. I have two lines (as in two physical US phones with a third spare). Instead of choosing "Use data only," I used the other number to send the verification text message to. When I popped in the local SIM card (Vodafone), instead of typing in my number, I typed in the other number. With the verification text received, I was able to setup Digits. So you should be able to setup Digits, on the iOS device using
Looking around on the Inter-tubes, I found a thread on reddit where folks claimed that they could validate Digits using alternate mobile numbers like Google Voice. So apparently, you don't need to send the text to the International phone number. All you need is just a US number that you can read texts on. So you can use your Android T-mobile phone to setup Digits on the iOS device.