Everything worth knowing, nothing that isn't.
How VoIP quality is measured: MOS
Mean Opinion Score (MOS) rates call quality from 1 (bad) to 5 (excellent); 4.0+ is toll quality, below 3.5 is noticeably degraded. Originally a human listening test, MOS is now estimated automatically (per ITU-T models) from measured latency, jitter, and packet loss. It is the single number carriers and platforms track to compare routes and diagnose a bad call path. See /answers/what-is-mos.
The impairments that lower quality
Latency (one-way delay above ~150ms makes conversation feel like a walkie-talkie); jitter (variation in packet arrival that garbles audio unless a jitter buffer smooths it); and packet loss (missing RTP packets heard as clipping or dropouts — above ~1% is audible). These accumulate across every hop, which is why direct, well-provisioned routes sound better than long ones. See /answers/what-is-jitter and /answers/what-is-rtp.
Codecs and the quality-vs-bandwidth trade-off
The codec sets the ceiling. G.711 is uncompressed toll-quality at ~64 kbit/s; Opus adapts bitrate and is resilient to loss; low-bitrate codecs save bandwidth but cap MOS. Transcoding between codecs along the path costs quality and adds delay, so matching codecs end to end matters. See /answers/what-is-voice-codec and /answers/what-is-sip.
How to protect quality
Prioritise voice with QoS/DiffServ so RTP is not queued behind bulk data; size jitter buffers to trade a little delay for smoother audio; pick loss-resilient codecs like Opus on lossy paths; and choose direct routes with few hops. Continuous MOS, ASR, and PDD monitoring surfaces a degrading route before customers complain. See /answers/what-is-asr, /answers/what-is-pdd, and /products/voice.
Where poor quality actually comes from
Most bad VoIP is not the codec — it is the network path. Grey or over-hopped wholesale voice routes add latency and loss at each transit carrier, congested access links starve RTP of bandwidth, and unmanaged Wi-Fi introduces jitter. Diagnosing quality means correlating MOS with the route and the KPIs (ASR, ACD, PDD) that reveal a weak carrier leg. See /answers/programmable-voice and /platforms/signaling-cdr.
What is a good MOS score for VoIP?
A MOS of 4.0 or above is considered toll quality — indistinguishable from a landline to most listeners. 3.5-4.0 is acceptable, 3.0-3.5 is noticeably degraded, and below 3.0 frustrates callers. G.711 on a clean path tops out near 4.4; the goal of QoS, jitter buffers, and direct routing is to keep the delivered MOS as close to that ceiling as the network allows. See /answers/what-is-mos and /products/voice.