"Ffonian nhw am ddim byd."
Translation:They will phone about nothing.
What does the English sentence actually mean? They will phone to no purpose? They will get a free phone call?
There are two posssible meanings, the first, as you suggested, 'they will phone for no purpose', the other 'they will phone (to talk) about nothing (of consequence).
To get a free phone call it would be ' ffonian nhw am ddim'
I've changed 'for' to 'about' to avoid the ambiguity.
Thanks for the comment.