A recent study in the Journal of the American Medical Association found that some brands of calcium carbonate do contain some lead, but far below the federal limit. Lead is mainly of concern for children and pregnant women taking calcium supplements. Lead occurs everywhere, and it's not possible to guarantee absolutely lead-free food or supplements. Some kinds of fiber do reduce calcium absorption somewhat. If you eat a large amount of wheat bran cereal for breakfast, for instance, you might wait and take your supplement at lunch.
Oxalates are substances that bind calcium.
If you consume the recommended 1,000 to 1,500 milligrams of calcium a day, however, such dietary interactions shouldn't be a problem. If you're on hormones, it's likely that your body will better utilize the calcium you consume. But it's still safest to aim for at least 1,200 milligrams a day. Make up any shortfall with supplements. Buy supplements by price.