Sounds like someone had an ACS in front of that 206e at one time. The Acs has 3 co lines and 8 ext. ports so it would be ext 10 thru 17 on the ACS processor instead of 10 thru 15 that the partner 3.1 processor has,it is 2 co lines and 6 ext.I don't know how to do the 3 digit extensions but but I did read about it on another support site, sounded complicated. I like 2 digits they are easier to remember and assign.