Not sure what you mean -- everyone knows that a phone call is not routinely recorded and there is no expectation that it be recorded when an investigation is on

Not sure what you mean -- everyone knows that a phone call is not routinely recorded and there is no expectation that it be recorded when an investigation is on and businesses routinely use a mix of phone calls and emails (and other means) for communication. Why should there be a "by default" expectation that you would increase retention just because there is an investigation in progress (which is why I asked if there is some explicit law/requirement for emails).