Also, ‘You could at least have called.’ That sounds a little more natural, emphasizing the ‘at least’

Yes, you can use these two sentences interchangeably with slight variations in emphasis: 1. You could have at least called. - This sentence emphasizes the action of calling as the minimum expected response. It suggests disappointment or expectation that the person should have made the effort to call. 2. The least you could have done was call. - This sentence also conveys disappointment or criticism, but it emphasizes that calling was the minimum action that could have been taken. It implies that the person didn't even do the minimum expected. Both sentences express a similar sentiment of disappointment or criticism about the person's failure to call, but they emphasize it in slightly different ways.

Building on what Izzie said, I would say the first one is casual/informal, the sort of thing I would say every day. The second option is more dramatic, and I would say it sounds stronger and angrier for that reason.

While yes, they can be used interchangeably, but the first sounds more casual, and the second sounds more formal, like you would use it at the office or a fancy meeting