Illegal to curse over state lines?

I work in a customer service call center, and I over heard one of my fellow agents, say it’s illegal to curse over the phone accross state lines. I have trouble believing this. I’d imagine if it were true it would a federal crime, state lines and all. Anyone know if this is true?

If it is true, I’ve broken it copious times. Regardless, it seems it would be impeding on the 1st amendment if true.

I seriously doubt there is any federal law about the exact words you can and cannot say on the telephone accross state lines. There are federal laws about harassment, fraud, threats, etc. that occur on the telephone accross state lines.