You can remember about 3-4 things (for about 20 seconds) and then they will disappear from memory unless you repeat them over and over. For example, let’s say you are driving in your car and talking on your cell phone (ok, you shouldn’t be doing that) and someone gives you a number to call. But you don’t have a pen handy, and anyway you are driving. So you try to memorize the number long enough to hang up from one call and dial the new number. What do you do? You repeat the number over and over (putting it back into short term memory each time, which buys you another 20 seconds). The interesting thing about phone numbers is that they are more than 3 or 4 numbers long. So they are hard to remember for more than 20 seconds.
We also tend to chunk information into groups that have 3-4 items in them. So a phone number in the US is: 712-569-4532. Three chunks, with 3-4 items in each chunk.