1. A control node in a cellular communication system, comprising: one or more communication interfaces configured to communicatively couple the control node to a first communication node and a core network, the first communication node comprising a vocoder comprising a voice encoder that is capable of operating in at least a subset of a plurality of predefined encoder modes; and a processing subsystem associated with the one or more communication interfaces configured to: during a call between the first communication node and a second communication node, dynamically send capability information for the voice encoder of the vocoder of the first communication node to a second control node for the second communication node via the core network in each of a plurality of Real Time Protocol, RTP, packets transmitted from the control node to the second control node carrying encoded speech encoded by the voice encoder of the vocoder of the first communication node for the call between the first communication node and the second communication node, the capability information for the voice encoder of the vocoder of the first communication node being indicative of the at least a subset of the plurality of predefined encoder modes in which the voice encoder is capable of operating.