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, has evolved toward the single-word voicemail—no hyphen, no space.
Most of the English-speaking world already favors the one-word form. In current British news publications, for instance, the ratio of voicemail to voice mail is about five to one. American writers are the laggards on this one; in current U.S. news publications, the ratio is still about one to one. This is probably due in part to the influence of the New York Times, which is notoriously still stuck in the 1990s with its spellings of internet-related terms (see also their spelling of e-mail where most everyone else uses email, their capitalization of Internet, their use of Web site for website, and so on).
If you’re writing for an audience that may be editorially conservative (e.g., a stodgy professor or anyone who reads the New York Times and puts stock in its style), voice mail might still be the safer choice. But voicemail is rarely questioned anymore.
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]