When you leave a phone message, or when you record your own OGM, it's time to get dramatic. Emphasize, and be clear and to the point, or else you'll mumble the most important part, or even a digit.
It makes me so sad to hear a message, or an OGM, that just cannot be understood. I once needed to reach a doctor's emergency line. The emergency number played a message about how to call the doctor's weekend number. That number was just thrown away in a rapid soft voice at the end of the message, and I think I was lucky to be able to hear eight of the nine digits. I reached the doctor by calling almost all ten of the possible numbers, in the course of which I got to talk to some very nice people, but that wasn't important then.
If your message includes a phone number, speak it twice! It doesn't matter how clearly you enunciate your digits; what matters is that a moment of noise can obliterate one of them. Give your listener a second chance.