There are a few scenarios where HD voice may not be available. POTS/PSTN Calls: Calls connected over the Public Switched Telephone Network (PSTN) do not support HD voice due to their narrowband codecs. Low Bandwidth: Network connections with high latency can cause VoIP services to gracefully degrade the call quality in favor of achieving a more reliable connection. Cellular networks: Not all cell networks support HD voice. Top carriers have only recently expanded support for HD voice calls happening outside of their network. We recommend installing the latest iPhone (iOS) and Android carrier updates to achieve the best possible calling experience.