It is illegal to curse on the telephone. Piss someone off and they will swear a warrant otherwise, no problem;
Curse at someone in a heated way and you could be charged with assault if the police hear it or the person swears a warrant, otherwise no problem.
Curse at the police and you are going to be charged with swearing in public, disorderly conduct or something more--virtually guaranteed.
Curse at a judge and you are in contempt of court.
The long and short of it is, don't piss people off and you never have to worry about it even if you are using profanity in my hood. Otherwise, there is no real line.
BTW I get to say profanity in court if I am quoting someone or soliciting info like "When you stated they said FU B, just so there is no confusion, is it true they said **** You ***** or did they literally say 'FU B'?" It still freaks me out to use the "N" Word but sometimes it becomes unavoidable.