I think you have the same problem I had in that the phase is the opposite sign; I'm still trying to figure out why. Note also in the paper, the authors plotted the "unwrapped" phase i.e. when phase goes above pi or below -pi, it jumps to -pi or pi (see how the authors have phase values of -4 radians; this is the same as having a positive phase value).
If you're still working this problem, I'd love to discuss with you.