A lot of people think that the employer's handbook has the force of law, including on the employer. This is mostly not true. If there is an actual state/federal law which explicitly says to look at the employer's handbook, then we maybe have an indirect legal requirement created by the handbook. CA vacation law is a good example. But there is no general rule that The Handbook equals The Law,
Past that, assume that there was no handbook. The employer's verbal policy would still be legal because there is no actual law saying it is illegal. Formal written policies are generally required for many benefit plans, and strongly recommended for compensation policies. But that is a very long way from saying that an employer's cell phone policy carries legal force.
The key here is that under Employment At Will, all terminations are legal unless a very specific law says that they are not.

