Answer:
Yes, you can. If your iPhone is jailbraked, you can install iLog via installer. It helps you manage your calls/call duration/sms.
you can get it here: www.ixtension.com
If your iPhone is not jailbreaded, please check this:
http://www.ipodrobot.com/blog/delete-single-recent-call-on-iPhone-without-jailbreaking.htm
If your phone is not jailbroken as is the case with the new 3.1.3 and baseband 5.12.01 and you can not jailbreak here is an alternate solution. The ipodrobot does not work if you have an apple computer and I have yet to find an alternative via that route. Additionally, this route is free, but only works if you are can set the circumstances.
First, plug your iPhone into your computer and synch it to itunes. Once synch is complete, right click (or apple alternative) on the iPhone under the devices on the left toolbar in Itunes. Go down to backup iPhone and select that option.
This part is important. Leave the phone plugged in. If you unplug and make the call then plug back in you risk creating a new file and overwriting the backup or having to use an older backup from a while ago that may seem out of the ordinary. Perhaps someone can chime in with a way to keep this from happening, but I do not risk it.
At this point, make the call or receive the call. Once the call is complete. Right click on the device again and this time choose the restore from backup option. The backed up file should appear as whatever you have named your device. The process of restoring should take about a minute. Everything will be there...except the call you made.