I think a lot of it is going to depend on your data structure and the purpose of your app. Will a phone number be enough to identify your users? Will you need any more information?
Phone number and passcode definitely provides a very simple interface which is more likely to convert visitors than turn them away (the way a longer more detailed process would be). I hate typing on my phone so the less I have to enter the better.
If you want more details you can always offer up a profile section to gather additional details once the user is registered and logged in. Some friendly prompt to get to know them better or something along those lines. The drawback there is some users will skip the profile.
So, keeping the login simple is a good goal but make sure it gets the information your app needs.