At CREDIL we are expanding our Asterisk out to service the entire floor. We didn't do our extensions particularly efficiently (numberwise), and I was thinking about ways to do them.
A really (math) geeky way occured to me: give employee number n the n+2'th prime (1-first prime, 2-second prime, first employee gets extension 3).
Then, if you need to have a conference call with employees number 4, 6 and 9, then you need to dial their product. Primes are 1,2,3,5,7,11,13,17,19,23,29,31,.. 4+2 = 6th prime is 11, 6+2=8th prime is 17, and 9+2=11th prime is 29. So dial 11*17*29 = 5423.
Primes are still in the 4 digits for the first 1000.
All multiples of your extension are yours to do anything you want with, and since the multiples times powers of 2 are never conference bridges, you have a lot of bits you can use to encode useful things. Want to call me and avoid ringing me? Okay, set bit number 2. Want to call me and never go to voice mail? Okay, set bit number 3... etc.