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.
Avoid background noise. Whether you have music playing in your office, or you’re sitting in a coffee shop, background noise can make it difficult for your customers to understand your greeting. Limit the noise around you when you leave your voicemail greeting.

Leave a specific request. What do you want your listener to do? Sure, you want them to call you back, but why? To answer a question? To set up an appointment? People will appreciate it if you give them specific actions for their call back. That way they’ll know they won’t be wasting a lot of time on the call back trying to figure out what you want.
-Now I lay me down to sleep; Leave a message at the beep. If I should die before I wake, remember to erase the tape.