Every time there is a phase change, the temperature of the system does not change because all the energy supplied to the system is used to break the bonds between molecules.
For instance, in the transition solid --> liquid (melting, so it is the case of the problem), the energy supplied during the transition is used to break the bonds between the molecules of the solid, so that the molecules are then free to move and the solid becomes a liquid. All the energy provided during the transition is used for this purpose, so there is no increase in temperature of the object.
