Cavalry Call lets you call in a carriage packed with Rooks when a fight turns against you. It is the end of the Ringleader carriage line.
Cavalry Call is entry 9 of 17 in the Ringleader branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £8,000, 150 Metal, 100 Leather. Before Cavalry Call unlocks you need Rook Carriage.