As any close follower of English could have predicted, the two-word phrase voice mail, which was the most common spelling when the technology was new, is evolving toward the single-word voicemail—no hyphen, no space. The two-word and one-word spellings are neck and neck right now, but voicemail will win, and we might as well accept it.
Examples
For example, these major English publications use the one-word voicemail:
Once, she left a panicked message on Jodi’s voicemail. [USA Today]
Mulcaire – as the world now knows – proceeded to hack the voicemail messages of public figures. [Guardian]
British authorities are investigating voicemail interception . . . [Wall Street Journal]
The following night Adam left a voicemail message to say that he was “rapt” and that he would like to see her again. [Sydney Morning Herald]
If you’re writing for an audience that may be editorially conservative (e.g., a stodgy professor), you may want to use the two-word voice mail. But voicemail is rarely questioned anymore.
