For business VoIP in 2026, the short version is simple: use Opus wherever both ends support it. Keep G.711 A-law for PSTN compatibility and legacy SIP interop. Avoid GSM and G.729 unless bandwidth is extremely constrained and lower call quality is acceptable.

Codec comparison table

Notes on the names

• G.711 is the ITU standard.
• A-law and μ-law are the two companding variants within G.711.
• In the UK and Europe, A-law is the normal legacy telephony choice.
• When people compare A-law and G.711, they are usually really comparing:
• G.711 A-law
• G.711 μ-law

Why Opus is the modern codec to use

Best quality per bit

Opus delivers far better audio quality than older narrowband codecs at the same or lower bitrate.

• At low bitrates, it is usually clearer than G.729 or GSM.
• At moderate bitrates, it provides wideband or better audio, which sounds much more natural than PSTN-grade speech.
• It scales from very constrained links to high-quality voice and conferencing without changing codec family.

Handles real-world networks better

Opus was designed for modern IP networks rather than old fixed telephony assumptions.

• Adaptive bitrate helps on changing links.
• Better resilience to packet loss and jitter.
• Lower delay options are available.
• Works well for both speech and mixed audio.

That makes it particularly suitable for:

• remote workers
• mobile and Wi-Fi users
• softphones
• conferencing platforms
• internet-based SIP and WebRTC environments

Better speech quality

Compared with old narrowband codecs, Opus supports:

• wideband
• super-wideband
• fullband

This improves:

• intelligibility
• natural voice tone
• listening fatigue
• overall call experience

In practice, calls sound less “telephone-like” and more like normal speech.

Open and current

Opus is a modern, open codec with broad support across current software and platforms.

This makes it a better long-term choice than older codecs that were designed around:

• legacy carrier constraints
• older DSP limitations
• historical licensing issues

Why A-law is still used, but is legacy

A-law remains common because it is the standard format for a great deal of traditional telephony in the UK and Europe.

Why it is still useful

• Near-universal support on SIP trunks, gateways, ATAs, and PBXs
• Simple and low-overhead
• Good interoperability with PSTN equipment
• Minimal CPU usage

Why it is legacy

• Fixed at 64 kbit/s
• Only narrowband voice
• No modern adaptation to network conditions
• Lower perceived quality than wideband/fullband codecs

So A-law is still a sensible compatibility codec, but not the best quality/bandwidth codec.

Why G.729 and GSM are usually poor choices now

These codecs were designed for times when bandwidth was much tighter and processing choices were different.

Why people used them

• Very low bandwidth requirement
• Useful on old WAN links
• Popular in earlier VoIP deployments

Why they are not attractive in 2026

• Noticeably lower voice quality
• More synthetic or “watery” sounding speech
• Narrowband only
• Worse performance for music on hold, prompts, and mixed audio
• Increasingly unnecessary on modern business internet connections

In short:

• G.729: efficient, but sounds compressed and dated
• GSM: old and rough by today’s standards
• Opus: usually gives better quality at practical bitrates, especially on modern endpoints

Summary

• Opus is the best modern choice because it gives the best balance of quality, resilience, and bandwidth efficiency.
• A-law / G.711 is still important for compatibility, especially with legacy telephony and UK/European SIP/PSTN environments.
• G.729 and GSM are ultra-low-bandwidth legacy options, but the quality trade-off is usually no longer worth it.