I never bothered with it either until one of my local customers rang me to say that 'Microsoft' were ringing all his numbers in rotation looking for 'customers' and could I block them.
So I worked it out and they were happy with that but I also discovered that I could put my name on the screen of the phone when I rang in even from a different number.
On earlier versions there are Reserved elements in the COT that need to be changed but for most systems the information above should work.
For reference here is a copy of the ones that I have blocked but you will have different ones I assume.
ADD-CLIMA:CLI,1,"12070486825","India",,,REJECT;
ADD-CLIMA:CLI,1,"1253820308","India",,,REJECT;
ADD-CLIMA:CLI,1,"16466885586","India",,,REJECT;
ADD-CLIMA:CLI,1,"2011772013","India",,,REJECT;
ADD-CLIMA:CLI,1,"442078909051","India",,,REJECT;
ADD-CLIMA:CLI,1,"79569","India",,,REJECT;