How can I see the call history from one month ago?
There's no way to see the true call history as a proper timeline through the iPhone, although with a bit of work it can be achieved however apps like PhoneView allow you to see your call history as individual, historical calls.

When you connect to the app, it downloads your call history for you, and you can then clear it on the iPhone without losing the history in the app.

Now, one of the things that the iPhone history does is try to group conversations into one event.  If you compare my call log from my iPhone and from PhoneView below.

This first image is the output from PhoneView, showing a list of all my calls for most of a week in February, date ascending.



The second image is a screen cap of my iPhone call history for the same time frame, dates descending just to confuse you.



As you can see, the iPhone does some grouping on calls, both incoming and outgoing, to the same number.  Note that 11 calls on the 10th, plus 2 each from the 12th and 13th, and finally one on the 14th appear against the call log for the 14th making a total of 16 calls.

Here's what the iPhone tells us about that grouping (thats by pressing the blue arrow next to the item)



So what does this tell us:


either you deleted the call log for that call (swipe to delete)
they called again, or you called them on a different date and the
call log is now on that date and you are looking in the wrong place
you accidentally assigned the number to an existing contact and
    you are looking in the wrong place