Does an employer own the data on a cell phone provided to its employee? If so, is there a cause of action against an employee who provides a copy of the cell phone to a third party without authorization?
An employer owns the data on a cell phone provided to its employee if the employer has made that a condition of employment. The employer might own the data independently (e.g. the employer created it). Or, the employer might own it if the employee created it in the course of their employment. It's not where the data is that determines ownership, it's who created it. It is also possible that an employee could create data on a phone which is not in the course of their employment, but was also contrary to company policy (don't put your data on our phone).
If you violate company policy on use of equipment, the simplest course of action that will follow is getting fired, or losing some company benefit. Conceivably you might get sued, but that would require more than simply getting an unauthorized text message.
If we assume that the phone contains company intellectual property, copying that content would be a violation of copyright or patent law, so the employee can be sued for that. If there is a specific employment-related agreement (a non-disclosure agreement), the employee could suffer other consequences including being sued for something.