Leaving a voicemail can still be a good way to grab the attention of a potential new customer. However, when it comes to leaving a voicemail, you only have a few seconds to actually grab the listener's attention. Most salespeople screw this up by leaving too much information and making themselves sound like a salesperson, which is the kiss of death when leaving a voicemail.
A good voicemail greeting is short and professional, lets people know that you’ll get back to them, and invites callers to continue engaging with a call-to-action. You should also show your personality if you’re in an industry or role that allows that. If your industry is more conservative, however, you’ll want to keep humor and personal touches to a minimum. A greeting Your name Your company A simple explanation for missing the call (e.g. you’re away from the phone or are on holiday) A rough estimate of when you’ll get back to the person An alternative person to reach out to (if you’re out of office) An alternative mode of communication (if you prefer email or text) A call-to-action such as “Leave a message” or “Send me an email at [email protected]”

You should acknowledge that right off the bat by apologizing that they couldn't reach you directly.
In a highly competitive graduate market even something like an unprofessional voicemail message could mean the difference between you getting selected for an internship opportunity or being rejected. There is no point in having an amazing interview and following up with a thank-you email to only let yourself down with an unprofessional sounding voicemail.