My oncologist didn't insist I take calcium but suggested it might be a good idea. I am on tamoxifen now but she would like me to switch to an aromatase inhibitor later and the AIs, because they block estrogen production, can lead to brittle bones. I'm sure though that taking a calium supplement even if you never go onto AIs is probably a good thing for your bones.
However, I did read somewhere on the internet that calcium supplements can inhibit the absorption by your body of tamoxifen. Can't remember the source of that information and haven't asked my oncologist if it's accurate but just in case I always make sure to leave a few hours in between taking them. I have my tamoxifen first thing in the morning and my calcium supplement later in the day. I just take one of those Caltrate supplements. I also take vitamin D (and a few other things.) Good luck, Janet.