Legal Question in Business Law in California

None. You are neither entitled to a phone number that your employer owns, nor to any privacy in the use of that account or phone. If you want privacy you have to get your own private phone on your own personal account. The law is very clear that employees have zero rights to privacy or anything else in their employer's business technology, including computers, email, and phones.

Mr. McCormick is right about the law. His conclusion is very likely right as well, but it might not be.

How the law applies in a given situation depends on the facts. It's possible that the company has acted in ways that give you more rights than you would otherwise have had. I would need more information before I could say whether you have a viable argument. That would require some fairly unusual facts. The most likely answer is that you have no right to the phone number, as Mr. McCormick says.