You have to establish damages to pursue a libel (written defamation) and slander (spoken defamation) suit. It does not sound like you have suffered any damages as a consequence of the March incident.
Your facts about the phone incident are a bit confusing, but it is unlikely you could be fired for this situation. Most employers will provide you at least three warnings before termination, although that is not required because California is an "at-will" employment state, which means that unless you have a written employment contract, you can be terminated at any time for any reason. Some lawyers will disagree with this last statement and argue that you must have notice, but employers generally only provide notice to avoid wrongful termination lawsuits. In any event, focus on performing your job, not getting into disputes with your co-workers and all should be fine.
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