I know that Sprint has LTE, but an iPhone 7/+ or 8/+ bought from AT&T does not support CDMA and CDMA support is required for a phone to work on the Sprint network. This may be why the AT&T rep said that talked to the OP told him that his phone would not work on Sprint/Virgin as the rep may have not realized that the AT&T model of the 6s/+ does support CDMA. I was just trying to come up with a logical explanation as to why the OP may have been getting mixed messages as to whether or not his unlocked 6s Plus would work on the Virgin/Sprint network.