I'm currently using one number for multiple devices. For the most part, it's working well. My feedback below is based on multiple usage scenarios. I hope it's useful for the T-Mobile team.
Device usage spans the following: Sony Xperia Z5 compact and Z3 compact (Android 6), iPhone 5s (iOS 10), Amazon Fire Phone (Cyanogen Mod 11), iPad mini - no SIM, Windows 10.
* Digits site has default server software page - https://mydigits.t-mobile.com/
* On additional SIM line, clicking "Manage Multi-Line Settings" does not properly load.
* Mobile App - Android - In some circumstances, the device number validation needs to bypass. Maybe there is a number associated, but the user only intends to use mobile data or wifi to access their account. This works as expected on a device with no SIM card, but the presence of a SIM card does not mean the user intends to use that SIM and it's associated data/number.
* Mobile App - Android - Related to the last item, the phone number entry does not allow the entry of international numbers. It would be a common need to use an international SIM to access digits via the mobile data and/or wifi. Realistic usage scenarios span both the usage of that international number or just it's data or neither (as mentioned in previous item).
* On an international SIM, the app is claiming that the phone's number is the digits number, but this is not the case.
* It could be desirable to allow logging-in the digits app on the primary phone/SIM associated with the digits number. This would encourage users to use the app for texting, calls, and it would unify messages over all devices.
* Web Browser - Windows 10/Chrome - Google contacts login does not persist. Even during the same session, keeping the page open, the contacts get lost and I have to log-in again.
* Ability to save login credentials. If it's always a single user's devices, they may never want to sign out.
* Settings option to disable deletion of user settings/preferences. The default behavior is fine, but probably not needed for someone who is only using their own devices.
Thanks for reading, and great job enabling the digits product!