Legal Question in Employment Law in Georgia

It sounds like the employer is enforcing work rules, and expects you to be available to work at non-scheduled times, if the employer notifies you by phone. And that you didn't answer the phone because you were at the ER (certainly understandable).

Do you have a union that represents you, and a collective bargaining agreement about work rules?

Does the employer have a written list of work rules, that was previously provided to you?

Ordinarily the employer has the right to enforce work rules but there are some restraints. Perhaps the employer has violated either a bargaining agreement or a published set of work rules. If you find something like this, you have a better chance of a claim.

Another possibility is that employer is retaliating against you for some other reason, and is just using this incident as an excuse. For instance, if you are female, and several males have missed coming in to work when summoned, but they were not disciplined. That is sex discrimination.

In any event it sounds like a harsh disciplinary action, given that you are rarely summoned for unscheduled work and were at the ER for a health-related issue. I would try to find out how other employees have been treated in similar situations.