1. Twilio Client requires a high speed and low latency network connection. Benchmark the network using pingtest.net or speedtest.net and aim to improve the scores.
  2. If your router supports QoS, prioritize UDP ports in the range 10000-20000, and the IP address of the agents making/receiving the calls. Tips on setting up QoS for VoIP in your network can be found here.
  3. Reduce network activity not related to VoIP or use a separate network for VoIP workstations.
  4. Chrome v38/39 has known issues of audio dropouts / one-way audio. Upgrade your browser or use Firefox and see if the issue persists.

For more information about improving call quality, please refer to this article.