Most people have multiple phone numbers, don't ask me why, but there seems to be a reason for this. In most cases we try to explain this with 'home', 'office' and 'mobile' numbers... but who tells me I can be home or in the office with my mobile phone. So the first challenge is always to choose the most appropriate number to call.
When I was a kid our home phone was only four digits (4764). My school buddies in the same town could easily reach me with these four digits. Others from out of town also needed an area code adding significant complexity. Today we all live in a global world and phone numbers have gone beyond understandable complexity. It's not the area code anymore that need to be added, but also the international access code.
Grab a random business card and try to call the number as it's written down. I don't know about you, but I most of the time fail. For instance my number would be +31 (0)6 55338183, but I can't dial the brackets ! When dialing from my office phone it's even worse. The + doesn't exist on the phone and needs to be replaced by 00 (how could I know that?) and even then it will tell me: The number you dial does not exist. The silly reason is that I first need to dial a 0 to make a phone call. This is all beyond all forms of reason.
I now also realize why star bloggers as Robert Scoble and Louis Gray can put their mobile phone numbers on their blogs and only receive a few phone calls a day.