Would this be another way of saying: "I still wonder how he managed to find my telephone number" ?
That would be singular, "il mio numero"; unless the speaker has more than one, which is actually rather common.
Could I also say: 'come lui abbia trovato...'?
That would simply be 'how he found...'. The question is asking for 'how he managed to find', which is slightly different
This is a very strange sentence in English
Ancora mi chiedo come DL abbia fatto a trovare = managed
In Italian, 'fare a (verb)' means 'manage to'. So 'fare a trovare' means 'manage to find'.
"I still wonder how he came to find my numbers."
"I still wonder what he might have done to find my numbers" is not accepted.
There is a mistake in the program. The correct answer is found and you cannot choose this answer. Only choice is find and that is not correct. So you cannot go further.