As we are not Apple, we can only offer our experiences; mine are as someone who managed the technical services section of a company in another industry. Phone calls to any big company are ineffective; even in our smaller organization, people were hard to reach by phone due to shift staffing and weight of calls.
As the person who had to deal with the complaint, particularly on about a specific employee, I preferred a concise business letter with the details. If a personal matter reaches the disciplinary stage, a letter has evidentiary value; a pink " "While you were out..." slip, not so much. A letter with documentation, good or bad, can become part of of an employee's permanent file.
If you are in the US (these are international English-language forums) I recommend mailing the letter certified mail, return receipt requested. That way you should know who received it and when.
That all we can do other than give the address:
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