They occurred because of the increase in subscriber - dialled international calls from countries which used a different mapping of letters to numbers on the rotary dial and .
The number of useful exchange mnemonics that were possible under the existing system was being exhausted .
They occurred because of the increase in subscriber - dialled international calls from countries which used a different mapping of letters to numbers on the rotary dial and because the number of useful exchange mnemonics that were possible under the existing system was being exhausted .