Please. 

If you are leaving it, you want me to call you back.  In order to call you back, I need to capture the phone number and it takes several extra steps on my end to find it in my contacts if you are there at all. 

Oh, and yes, people still do use the voice calling features on their phones to communicate.

As I am not blessed with a brain that captures and catalogs all information heard once for instant retrieval, I am going to need to write it down.  I may get all the digits but want to make sure so repeating it for me again slowly will ensure I will call you back quickly and keeps me from having to listen to the entire voice mail again to get the number, hopefully.  This is especially true if you are not someone with whom I regularly communicate (ie, not in my contacts/saved numbers).

I always do this because I want to make it as easy as possible for people to call me back.  Try it out and see what you think especially if you are leaving a voice mail for me.