It would depend. If the voicemails were pertinent to the ongoing litigation, the counsel for the party responsible for the legal hold might be required to ensur

It would depend. If the voicemails were pertinent to the ongoing litigation, the counsel for the party responsible for the legal hold might be required to ensure those voicemails are retained. The legal hold isn't "preserve anything that's convenient to retain". If they can't be retained, they shouldn't be left, and it's the job of the legal team to make sure that's conveyed.