Here is a quick tip suggested to me by Ernest "Ernie the Attorney" Svenson that is helpful if you are ever trying to remember who you recently talked to on your iPhone (perhaps as you are doing your billable time entries). In the Phone app, tap the "Recents" button at the bottom to see a list of all of the people who you recently called or who called you. For older entries you just see the day of the call listed, but tap the blue arrow on the right of each entry to see the specific time of the call. Unfortunately, there is no way to display how long the call lasted once the call is over, but this might be enough to jog your memory and remember who you talked to and what you talked about. [UPDATE: I haven't tried this myself, but Martin points out in a comment to this post that you can use a script called alllog2ical.rb, available here, to get more info such as call duration.]
For some entries you may see a number in parentheses next to the person's name, indicating that there were multiple calls with that person. The time or date of the most recent call is listed in Recents, and you can tap the blue arrow to see a list of the precise date and times of the calls.
I'm not sure how many recent calls are stored. I vaguely remember reading somewhere that the iPhone logs the last 80 calls, but mine doesn't seem to have quite that many. Suffice it to say that dozens of the most recent calls are logged.
The call log will be greatly improved when Apple releases the iPhone Software 3.0 this summer. But even the current, limited, version of the call log is a useful part of the iPhone that you might forget is there.
