You can have both selector codes and direct extension transfer at the same time, but the selector codes cannot conflict with the 1st digit of the extension numbers.
The extension numbers are fixed in the hardware modules, starting at 10 and going up from there.
So, you can have "dial the extension number now, or press 3 for billing....." assuming your extensions range from 10 to no higher than 29. If your extensions range into the 30's, then you must start your selector codes with 4.
Leave room for future expansion!!