Brainstorm new tasks for a split story. Try Phase 2 again. The Customer can add in another story and see if the team can fit it in. All tasks are accepted. Customers start writing tests.
Programmers implement tasks.
After the team has taken the trouble to break it up into tasks and estimate them, they have better information about the actual work involved. The release plan said this story could be done. It might well affect the schedule. You hope there are other stories that come in quicker than you asked to make up for it. If not, at least you know now.