Phonetic clue of characters in languages or dialects other than Mandarin? Many characters are made up of a semantic and a phonetic part.

Is the phonetic part only valid with regards to Mandarin pronunciation?
Or not even for Mandarin in all cases?
Or does it work with for example Cantonese as well? That is, are Chinese dialects at least partially 'parallel'?

I suppose the phonetic clue does not help a Japanese or Korean learner. Am I right?
Generally yes. But I want to mention that Mandarin was also a dialect. So I'm not agree that the dialects are variations of Mandarin, I think that they are parallel. Most dialects are more ancient and closer to the ancient Chinese than Mandarin.