So essentially, back to and, in fact, never moved forward from the sheer unlikelihood of the caller coming forward, discussed at length on previous threads!
Just to recap, if this phonecall was an innocent one (ie. not concerning MM) and the caller knew themselves to have no knowledge of and nothing whatsoever to do with MM's disappearance, then s/he would surely have come forward by now? Since s/he hasn't, the logical conclusion is that it was and is someone who has no interest in opening themselves up for scrutiny.
What will Wolters do if this caller never comes forward? Will he never issue an arrest warrant? Never interview CB? Shrug and say 'Well, we tried but...'?!