Typically, most of these services are uniform—i.e. include the same tools and features (send a script, they record it in a tone you approve, they send it as a file, you supply any feedback, etc.); however, there are some services that go above and beyond. For example, VoiceOverPro, is a single voice professional who records 30 second voicemail messages based on users’ scripts. This is standard; however, users also have options like adding music, recording up to 60 seconds (or 150 words), rush, 2-day turnaround, and even same day delivery.
From the Calling User Portal, click Voicemail. 3 4

Pro Tip: Smile while you’re recording your greeting and your voice will sound pleasant.
The best text-to-speech voices don't sound robotic at all. Do a "text to speech" search in Google to explore your options. Type in your script and presto! You'll get an instant greeting with a natural sounding voice. You can select from a broad spectrum of male & female voices with any local or foreign accent you choose. LinkedPhone offers text-to-speech built right into our software. It's super popular and our users love it!