Memory---Short-term

The human brain is a fascinating organ, but what I find even more fascinating is the brain's ability to remember things, also known as memory. We have three systems of memory, sensory, short-term, and long-term, though researchers do believe there are other memory systems as well. Each of these memory systems differs based on two pieces. One of these pieces is span, which is the amount of information that can be held and the other piece is duration, the amount of time that information can be held.
All of the memory systems listed above are quite intriguing, but I am especially interested in the short-term memory system and its abilities. The short-term memory is said to have duration of up to 20 seconds, but usually tends to be closer to 10-15 seconds. Many people, I can admit that I am one of them, may claim they have short-term memory when they can't remember an activity they recently participated in or what they did a few days ago. If that were the case short-term memory wouldn't be so short.
The short-term memory system is said to have a "magic number" when it comes to its capacity. The magic number is claimed to be seven plus or minus two. I can agree with this claim from my experience as a test subject years ago. I, along with some of my fellow classmates, was shown many lists of numbers and after we were shown each list the instructor would tell us to write down as many numbers as we could remember. It was easy in the beginning, as there were only a few numbers to remember, but as the lists got longer than seven, we started to struggle to remember what came next. This holds true for other experiments as well and is the reason why many phone numbers have seven digits. It does mention in the text that some European phone numbers are a little longer, but nothing more than nine digits. I wonder if they have as many phone numbers memorized as the average American with the slight addition of digits.
I am excited to learn more about the human memory and would really like to know how the memory systems of those claimed to have photographic memories operate as well.