method name removNb seemed unnecessary, any reason you didn't name it something clearer like removeNumbers?

Thank you for this, didn't understand the requirement until I read this!

The wording of the kata is a little off, I'd suggest getting a native English speaker to proofread it

I think they forget to write code for others to read. If m jogs their short term memory that's good enough apparently

For the inclusiveness and self-awareness (and professionalism), this kata should specify that the phone number format being used is the North American Numbering Plan https://en.wikipedia.org/wiki/North_American_Numbering_Plan

The naming is quite poor for this kata. DRoot as a classname is a pointless abreivation which sacrifices clarity for no gain as DigitalRoot is an acceptable class name. digital_root as a method name is worse because it bucks the Java convention of using lower camel-case for method names. Otherwise it was good and fun to solve!

why are h and l used as variable names? do they stand for something?