In a telecommunications network including at least a user device and a network node separated by at least a packet-switched part of the telecommunications network, the user device including a primary jitter buffer having a constant packet play-out rate, the network node including a secondary jitter buffer, incoming packets destined for the user device are received and passed through the secondary jitter buffer of the network node downstream towards the primary jitter buffer of the user device. The departure times of packets passing through the secondary jitter buffer of the network node downstream towards the primary jitter buffer of the user device are monitored. On the basis of the monitoring and one or more known characteristics of the primary jitter buffer, an estimate of a current state of the primary jitter buffer is maintained. Operation of the secondary jitter buffer is dynamically controlled according to the maintained estimate.