A method of adapting payload transport between a mobile station (MS) and an unlicensed-radio access network controller (UNC) using the Real-Time Transport Protocol (RTP). During the activation of a traffic channel, the UNC and MS negotiate an initial sample size for the payload to be carried in each RTP packet. Thereafter, if network conditions change to the point that a different sample size is needed, another negotiation is initiated to change the sample size for the ongoing session. The UNC may suggest a sample size, and the MS selects the closest sample size that the MS can support and uses the selected sample size in RTP packets that it sends to the UNC. The UNC checks the received RTP packets and automatically adopts the sample size utilized by the MS. The UNC and MS may also negotiate changes to redundancy coding and other parameters that affect voice quality.