—As IMS becomes more available to academia and industry, the requirements for IMS clients are growing faster than ever. From all IMS components, IMS clients are vital components for the success of overall services provided by the IMS. Most of ongoing research is focusing on adding more services on clients side, none has been done for voice over Internet protocol (VoIP) quality adaptation so far. Since the end user is the only one to perceive the quality of services provided by the IMS network, this paper reports a testbed that demonstrates the concept of adaptation of VoIP system based on an open IMS core and open source mobile terminal.