You must have a feature access code assigned... Just go into the Feature Access Codes for that feature and change it to something you are not using...
My real issue here is the numbering scheme... Are all your extensions 3XXX? If so, most techs at the time of install will renumber the FAC's accordingly as the default scheme in the 5000 is 1XXX numbers are extensions, 2XXX are hunt groups, CRA's, and other system stuff, 3XX are feature codes, and 4-9 are for system features.
It is odd it would let you make an ext 3304 if 330 is a feature access code, the 5000 does not allow for conflicting digit plans.