Extensive characterisation of subsystems such as detectors and their curing, preamplifiers, filters, on-board secondary standard sources and cryomechanisms were the basis for the understanding of the assembled instrument and the development of a dedicated CALibration FACility for ISOPHOT. In the CALFAC the instrument was fully calibrated and the Astronomical Observing Templates tested. The high reproducibility of measurements was confirmed. This allowed the in-flight performance to be predicted to within a factor of 2. The operation of replica detectors and the stand-by of an operational flight spare unit after the flight model delivery allowed the development of countermeasures for a few non-ideal properties prior to launch and during the mission.