At what stage of the system development project (for example feasibility study, requirement analysis, etc) would a prototype be useful as means of reducing the following uncertainties?

two Scenario

(1) A computer system is to support sales office staff taking phone calls from members of the public enquiring about motor insurance and giving quotation over phone.

(2) There is a proposal that the senior managers of an insurance company have personal access to management information through an executive information system installed on personal computers located on their desks. Such a system would be costly to setup and there is some doubt about whether the managers would actually use the system.