something like this works

select SUBSTR('ATM3/0/0', 1, INSTR(TRANSLATE('ATM3/0/0', '0123456789', '^^^^^^^^^^'), '^') - 1),
SUBSTR('ATM3/0/0', INSTR(TRANSLATE('ATM3/0/0', '0123456789', '^^^^^^^^^^'), '^'))
from dual

the ^ character I have used is any character that is definately not going to be in the string you pass it.

Mike