Heh, I had an unfair advantage: I didn't trouble to familiarize myself with the WBC.
But by my reasoning, the difference would be legal precedents stating that pickets are legal (irrespective of what you use them to promote) and repeated unsolicited telephone calls are not (irrespective of what you say).
That's not entirely true. You can't write "God Hates Blacks" (per the Racial Hatred Act) but you can write "God Hates Fags". There seems to be no logical basis for this difference.
To clarify: are you talking about the U.S. or another country (a moment on Google suggests maybe Australia?). I'm almost certain that writing "God Hates Blacks" (or waving a sign saying that, etc) is perfectly legal in the United States.
But by my reasoning, the difference would be legal precedents stating that pickets are legal (irrespective of what you use them to promote) and repeated unsolicited telephone calls are not (irrespective of what you say).