Given the difficulty of this exam, practically the entire exam could be considered a trouble spot. However, the one area that seems to trip people up more than anything else has to do with normalization.
Microsoft expects you to know how to convert phone numbers into normalized dial patterns. For example, a user might dial a phone number using one plus the area code, an area code without the one, dial as if it were a local number, or dial an extension number. Regardless of how the user dials the number, you have to use normalization strings to convert the phone number into a format that OCS understands. These normalization strings tends to be cryptic, so make sure that you devote plenty of time to understanding how they work.
There are virtually no study resources available for this exam. Your best bet (besides having plenty of hands on experience with OCS 2007 R2) is to first take exam 70-638 TS: Microsoft Office Communications Server 2007, Configuring. Passing this exam will ensure that you have mastered the basics of OCS.
Another helpful thing to know is that Exam 74-404 is very similar to Exam 74-924. Exam 74-924 deals with OCS 2007, while Exam 74-404 deals with OCS 2007 R2. Therefore, if you have trouble tracking down study resources for Exam 74-404, you might have better luck looking for 74-924 related study materials. The two exams obviously are not identical to one another, but there is a lot of overlapping material.
Recommended Study Resources
One of the big challenges with regard to the 74-404 exam is that it is extremely difficult to find reputable study materials that are specifically designed for this exam. Even though 74-924 is the older version of this exam, it is difficult to find reputable study materials for that one as well. As of the time that this exam profile was written, the resources listed below were the only ones available on Google Shopping.