Mammograms can and do detect calcifications and ductal carcinoma very well. There are, however, many causes of breast calcification and most calcifications in the breast are benign. A stereotactic biopsy may be suggested if there are worrisome calcifications on a mammogram.
Ductal Carcinoma in Situ - DCIS - is stage 0 breast cancer and often presents as calcifications on a mammogram. We do know, however, that some DCIS is not calcified and can only be appreciated on contrast enhanced breast MRI.