It was illegal, from about 2021 through about 2025, for employees of financial firms to text or WhatsApp about work on their personal phones. The theory was that US securities regulations require firms to keep copies of “all communications sent” by their employees, “including inter-office memoranda and communications,” that are “related to [their] business as such.” So if a financial services employee sends an email to a client, or to a colleague, her firm needs to retain a copy of that email. And if she texts a client or colleague from her personal phone, then the firm won’t have a copy, which violates the law.
This theory was essentially invented by Gary Gensler, who was the chair of the US Securities and Exchange Commission at the time, and I always thought it was a strange and aggressive interpretation of the rules. But you can understand why the SEC liked it: