This paper describes an effective H.264 rate control algorithm focused on low bitrate, low delay, mobile applications. This new method, based on the joint video team (JVT) rate control, is able to deal with variable bitrate channels, and variable frame rate, variable GOP size environments, estimating the available channel resources from a generalized client/server model. The results obtained show a consistent behavior in mobile, multimedia scenarios, with fast and robust tracking of the target bitrate under adverse channel conditions. The implementation of this algorithm in a novel P2P-based videotelephony application for 3G networks at Telefoacutenica Investigacioacuten y Desarrollo has proven to achieve a noticeable quality of experience improvement for the final user, who is given the ability to actively optimize the use of the available network resources.