Do you think that would be an improvement?
In books, “voicemail” is gaining but “voice mail” is still the dominant form. Books do tend to lag a few years behind Internet usage, but the difference is large enough that I favor keeping voice-mail.
For a borderline case in terms of popularity, the hyphened form is preferable, because it's easier to parse. So this speaks in favor of voice-mail too.
On the other hand, the tag name on other Stack Exchange sites is always unhyphenated: Stack Overflow, Android, Apple, Windows Phone, Sound Design. That's a good argument for going with the flock (even if I think the flock made the wrong choice). Even if we end up sticking with voice-mail, I've created a voicemail synonym for the sake of migrations.