Phone Logs

This morning I was watching the news, and they mentioned something about how Martha Stewart supposedly changed her phone logs. I also remember several other occasions when government officials or businessmen were required to turn over their phone logs to investigators.

Is there some sort of law or regulation that requires phone logs to be kept? Or is it simply a common management practice? I worked with several Vice Presidents and Directors at my old company, and never saw a formal log.

At the company I used to work for, we had to keep phone logs for conversations that could result in some liability. For instance, we had to write down everything DigSafe would tell us. We were told that they are legal documents of the same sort that our field books were. AFAIK, it was a company policy, not a legal one, but IANA Lawyer.