^ Not always necessary. While it is the case that one remembers their most frequently used number, there are cases when secondary numbers become hard to remember. Like I had this SIM which was a 'test' number which I used for only about a week before setting it aside. When I felt the need to use it about 4 months later, I had a hard time finding out my number as there was no balance, I couldn't call anyone, nor reach customer care without any balance. Thankfully, this plan had free SMSs, which I used to find out what the number was