A new visual servoing technique based on 2D ultrasound image is proposed in order to control the motion of an ultrasound probe held by a medical robot. In opposition to a standard camera which provides a projection of the 3D scene to a 2D image, ultrasound information is strictly in the observation plan of the probe and consequently visual servoing techniques have to be adapted. In this paper the coupling between the ultrasound probe and a motionless crossed string phantom used for probe calibration is modeled. Then a robotic task is developed which consists to position the ultrasound image on the intersection point of the crossed string phantom while moving the probe to different orientations. The goal of this task is to optimize the procedure of spatial parameters calibration of 3D ultrasound systems.