A mobile terminal in a wireless communication network detects congestion and lowers its vocoder source rate in response, to help alleviate the congestion. The mobile terminal may detect the congestion in a variety of ways. The mobile terminal may monitor bandwidth allocation in the network, such as by inspecting the UL-MAP and DL-MAP of an IEEE 802.16 OFDM Physical Layer Frame header, for symmetric allocations. Congestion may be defined by the number of symmetric allocations exceeding a threshold. The congestion threshold may be programmed into the mobile terminal, or may be transmitted by the network. The mobile terminal may infer congestion by being repeatedly granted less bandwidth than requested. The network my explicitly indicate congestion, in a MAC message or by setting a congestion flag in one or more voice data frames. The congestion flag may comprise a reserved encoding of the AMR header Frame Type field.