As an alternative you can order a code generator but for DKB that requires a paid debit-card. ING disables the phone app if you use a code generator. You cannot have multiple 2FA.
They tried to force me to add my phone number, but when you talk to a Rep you can get out of that too...
Germany likes to think that they belong to cabinets and powered with internal combustion engines. Internet was a new land in 2013. So every user-friendly feature has to be shoved into Germany's throat by EU (especially banks and insurance). The usual reaction from German companies is to wait until the last moment and then hire a law / consultancy firm to implement required changes as badly as possible.
