It's not as simple as that.
The call(s) weren't intended as malicious, the odd f-word doesn't make it particularly obscene and there was no slander or lies. I cannot see a case being made.
I wonder tho what would have happened if an 'ordinary' tax paying member of the public had made the same call to a celeb...? I'm guessing that they would have at least been investigated by plod if not spent some time in jail...
It seems that celebs are immune from conviction... they can openly take Class A drugs while 'ordinary' proles are arrested on TV (road wars, etc.) for smoking a joint...
With hindsight Gary Glitter must wish that he'd released record every now and again thus ensuring immunity from prosecution...