For most UK PSTN-facing SIP services, prefer G.711 A-law when it is enabled on the UKDDI account because it is widely interoperable and avoids unnecessary low-bitrate transcoding. Use G.722 for end-to-end HD voice and Opus for compatible WebRTC/softphone paths; keep G.729 for constrained links only when every required leg supports it.
Settings used in this guide
These are public service values, not your password or account-specific authentication ID.
| Approved codec order | Confirm in your UKDDI activation email |
|---|---|
| Media security | SRTP |
| DTMF method | Confirm in your UKDDI activation email |
Start with interoperability, not the longest codec list
A codec converts audio into packets. Both endpoints must agree on one codec, and any network element forced to convert between codecs adds processing, delay and another possible quality loss.
The live UKDDI account setting is Confirm in your UKDDI activation email. Do not enable or prioritise a codec merely because the PBX lists it; it must be supported on the carrier path and appropriate to every endpoint that needs the call.
G.711 A-law is usually the cleanest starting point for UK telephony where the carrier profile enables it.
G.722 gives wideband speech at a 64 kbit/s encoded rate when the full path supports it.
G.729 reduces payload rate but trades quality and can trigger transcoding; use only when supported and justified.
Opus adapts well across speech/audio and network conditions, but PSTN interworking usually needs another codec or transcoding.
Compare the four common choices
| Codec | Encoded rate | Best fit | Main caution |
|---|---|---|---|
| G.711 A-law | 64 kbit/s payload | UK PSTN and reliable business LAN/WAN paths | Higher total bandwidth than compressed codecs |
| G.722 | 64 kbit/s payload | End-to-end wideband/HD speech | Falls back or transcodes when a leg is narrowband |
| G.729 | 8 kbit/s payload | Genuinely constrained compatible links | Lower fidelity and more processing/transcoding risk |
| Opus | Variable, 6–510 kbit/s in RFC 6716 | WebRTC, softphones and adaptive IP audio | Not a universal PSTN trunk codec |
IP, UDP, RTP, Ethernet/VLAN, TLS signalling and SRTP overhead are additional. Packet interval, IPv4/IPv6 and tunnel/VPN headers change the real per-call figure.
Choose a short, deliberate priority order
- 1Read the carrier profile
Start with Confirm in your UKDDI activation email from the UKDDI activation details. Unsupported offers can cause rejection or unwanted transcoding.
- 2Map every call leg
Include handset, PBX, SBC, WAN, UKDDI and the PSTN or application at the far end.
- 3Prefer the common codec
Put the codec that can remain end-to-end near the top; for a UK PSTN-facing route that is commonly G.711 A-law when enabled.
- 4Keep fallbacks intentional
A long offer can create inconsistent negotiations. Enable only tested codecs and document why each fallback exists.
- 5Test features as well as speech
Verify Confirm in your UKDDI activation email, hold music, recording, voicemail, transfer, conferences and fax policy where used.
Plan bandwidth with headroom
Calculate simultaneous calls using the complete on-the-wire rate, not only the codec payload. Add headroom for bursts, other applications, VPN/tunnel overhead and both directions. Quality of Service cannot manufacture bandwidth on an already saturated access link.
- Measure packet loss, latency and jitter during the busiest business period.
- Avoid Wi-Fi for fixed desk phones where a stable wired path is available.
- Use call admission limits so the next call does not degrade every existing call.
- Check recording/transcoding CPU capacity separately from internet bandwidth.