Voice mail services are very useful when the person is not ready to receive a call or when unavailable but it costs many of them and you need to have it enabled. In case you want to send a voice mail in future , the facility is not there.  

Call to future is a online service which allows you to send a free and scheduled voice mail with just one restriction, it doesn’t record your voice, instead it asks you type in which is played by using the text to speech engine. So the receiver gets it like a call ( like Airtel in India has voice sms now ) with your number displayed on it. 

Free scheduled voicemail

They record your email id and you are allowed to schedule it for a particular date. However you can not use 

• Phone Number To Dial and CallerID to be the same number
• Phone Number To Dial and CallerID cannot start with 1, 800, or 555
• Max number of calls in any 24-hour period is 3.

Related :  Send free message to several countries without any signup