Legal Question in Criminal Law in California

How is the FCC going to regulate a cellphone? Also, she was not convicted of breaking the law.

The FCC doesn't "issue" cell phones. People just buy them in stores and then contract for service with a provider. The FCC does issue telecommunication licenses, but you don't need a license to have a cell phone. The FCC thus has no authority to do what you ask. And even if it did, an individual's complaint about another individual likely would not persuade it to use that authority. Such a request would be like asking the government to prevent someone who wrote a threatening letter from ever buying another pen.

It's not even clear that your boyfriend's ex actually threatened you. A threat has to either be directed to you or made in a way that the maker should reasonably expect will be relayed to you. She left the message on your boyfriend's voicemail, not yours. I would need to know more about the facts before I could say whether she should have expected you to find out about it.