Also from my reading of Endsong...my theory is when the Phoenix is consuming it's vessel that voice is fake Jeans. Because it copied her brain patterns and emotions when it replicated her. So effectively Emma in Endsong is being possessed by fake Jean Grey. So when Jean in Endsong is consumed she's possessed by fake Jean from Phoenix Rising.
Warsong...weakens the argument a bit because that voice talking to Celeste didn't show a lot of Jeanisms.