I agree with Robert, that the suppliers must be contacted in the Project Planning Phase as this is the phase where the PM has to make an educated guess of the staff, resources and equipment needed to complete the project. He needs to communicate and plan procurement activities, and make contracts for any 3rd party suppliers. In short, this is the phase where a clear project roadmap for the project is set. PM needs to prepare a comprehensive suite as per the requirements of the project. Apart from that, contacting the supplier also involves many steps such as: tender process, a statement of work, request for information and request for proposal which i feel will be difficult to carry out during the third phase. Because in the implementation phase, the project plan is in full motion. And it is more of a maintaining control and communication. Progress is continuously monitored and appropriate adjustments are made in accordance with the original plan.