Can I ask if you have rejected the offer already? Is calling or emailing better?
I think I'm gonna have to reject one offer very soon and I don't really know how to approach it.

Rather than wasting the interviewer's time, I feel more guilty on wasting the HR person's time cos they called me a few times so the rejection feels more personal.... And also I feel guilty that by stalling for so long, another candidate who could have gotten the role will not....