As the odds of receiving a call back from your voicemail message are already low, you need to be very specific about what you say in your voicemail to give you the greatest chance of getting a call back. What you absolutely do NOT want to do is make yourself sound like a salesperson making a cold call. Instead, you want to sound like a very confident business colleague who deserves the respect of getting a call back.
Repeat both at least twice during your voicemail. With regards to your telephone number, pauses in your speech will give your listener a chance to catch up and not miss any details. For example, say your telephone number this way “1-2-3- pause -4-5-6- pause -7-8- pause -9-10″. If you need to leave an email address, spell it out.

When do you leave a voicemail? This is largely dependent on your call sequence and cadence. It is recommended that you make several call attempts in order to engage the prospect on a “live call.” After you have done this and your attempts have not been fruitful, then you leave a voicemail.
In the U.K. and most of Europe it is considered good manners to leave a slightly longer answering message as it gives the person time to adjust to the fact that they've been put through to an answering machine, compose a message in their head and then get ready to speak. It also leaves the caller with the impression of someone who is calm, composed, polite and not rushing around like a chicken without a head.