It is critical that you respond within 24 hours to any communication from an employer regarding a callback invitation or job offer. (You do not have to accept or decline within that timeframe, but politely acknowledge receipt and let them know you will be in touch as soon as you make a decision.) Non-responsiveness is unprofessional, and inconsiderate of the firm's time. If it is more comfortable or logistically feasible to respond to a voicemail by email (ex: while you're at work or not in a location where a call can be made), feel free to do so.
Please contact the CDO if you have any questions, and remember to affirmatively withdraw your candidacy from any firms that you have decided you will not be accepting an offer from.