If I'm calling a friend who I know has my phone number and I have to leave a message, I won't leave my phone number in the message. If I'm not sure if they have my number, or if I'm giving them a different number to call me back at, I'll leave the number. And if I'm calling a business and leaving my phone number on an answering machine or voicemail, I'll leave my phone number. In each case, I'll include the area code, and I will pronounce each number, saying it slowly and clearly.
APPARENTLY, not everyone thinks this way. I can't tell you the number of times I've listened to voicemail messages where people have left a phone number, and they just ramble the phone number like it's some kind of contest to see how fast and jumbled they could possibly say it. They're not even saying the phone number with the same pacing as they've left the message. The message will be left in regular conversational tones, but when it comes to their phone number, they speed through it like they have someone chasing after them who's about to pounce so they really need to get off the phone. In almost all of these cases, I don't already have their phone number so I actually needed them to tell me. So, they either knew I didn't have the number, or they were just leaving the number in case I didn't have it. But in either case, they're still giving me the number for a reason, right, because they think I might need it? So why not say it at least clearly and slowly enough that if I actually *did* need it, I could actually understand them and being able to write the number down? How does mumbling it at a mile a minute help me to get that number?
There was this one time at work, and I am *not* exaggerating, where I had to play the message over and over again, because I could only make out one number at a time. Our phone system at work now has the ability to slow down messages, so there have been other occasions when I've replayed a message but slowed down so I could understand the phone number they were leaving me.
If you want me to call you back, why wouldn't you want me to be able to discern the phone number you're leaving me?