Under a construction management (CM) contract, should the CM firm's responsibilities normally begin at the construction phase, the design phase, the planning phase, or the conceptual phase?

Conceptual or planning design phase.

Explanation:

Construction management have following phases

Phase 1 : Conceptual or planning design phase.

Phase 2 : Engineering and design phase.

Phase 3 : Procurement phase.

Phase 4 : Execution phase.

Phase 5 : Operation and maintenance phase.

These all phases running in a proper sequence. So at the begging of construction the first phase is conceptual or planning phase.