It would really depend on retention time. If they check their email on a regular basis, and everyone (most users) delete the voicemail after listing to it, then unless (perhaps) you are going cheap with a 120Gb SSD, it shouldn't be an issue. Even with a drive that small, I'm not sure you'd run into a problem. The price of an SSD has dropped so much recently (120Gb =~$19 US). I wouldn't consider buying anything smaller than a 500 Gb.
Keep in mind that when doing a backup, and also backing up voicemails, which your users will thank you for if your system crashes, your backup storage space will need to be larger too.
You could start with a smaller drive, if you have one on hand already , keep an eye on it, then if you find it filling up, clone it onto a larger drive.
