The Ninth Circuit has ruled that even if your cell phone is owned and the plan is paid for by your employer, that employer cannot read your text messages. The court ruled that an Ontario police department violated an officers right to privacy when they obtained his text messages from the cell phone company to see if he had been writing personal text messages. They also ruled that if a company outsources its email service (no company server), that email is private as well.
This, despite the fact that the department had a policy clearly stating that employees should have no expectation of privacy and that the text messages and emails might be monitored. The kicker- despite that policy, they told the officer that if he paid the overage fess, his texts would not be audited. This created a reasonable expectation of privacy. If they had told him that in subsequent months they were going to audit the content of the messages, that might have been all right. The key- he had a reasonable expectation of privacy, and the search was unreasonable.
Clearly he did expect privacy- he had been writing sexually explicit messages to his wife.
Remember, that this is the 9th Circuit. This may lead to a different result in the 5th Circuit. But the Ninth Circuit, having jurisdiction over silicon valley, has a lot of experience with adjudicating tech cases
Source, Quon v. Arch Wireless Operating Co.