G.729 compresses speech to an 8 kbit/s payload, but RTP/IP overhead remains and compression can reduce fidelity. Use it only when UKDDI and every necessary call leg support it and a measured bandwidth constraint outweighs transcoding and quality costs.
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 |
The 8 kbit/s headline is not the whole call
G.729 codes speech at 8 kbit/s, far below G.711’s 64 kbit/s payload. However every packet still carries RTP, UDP, IP and link headers, so the total saving is smaller than comparing 8 with 64 alone. VPN, SRTP and IPv6 headers add more.
More simultaneous speech streams can fit on a genuinely constrained link.
Lossy compression, packet-loss sensitivity, codec processing and possible transcoding can reduce quality or capacity elsewhere.
Use G.729 only for a proven constraint
Do not choose G.729 to compensate for an unstable circuit. Compression reduces load but does not correct packet loss, excessive jitter, latency or a poor Wi-Fi radio path.
- The live UKDDI profile explicitly enables it: Confirm in your UKDDI activation email.
- The endpoint, PBX/SBC, recording and application chain can all support or intentionally transcode it.
- WAN capacity cannot reasonably be increased and measurements show bandwidth, rather than loss/latency, is the limiting factor.
- The business accepts the speech-quality trade-off and has tested IVR, voicemail, recording and conferences.
Account for transcoding and capacity
If the carrier leg, handset leg or an application uses a different codec, the PBX/SBC may transcode. That consumes CPU or DSP resources and each lossy encode/decode stage can further reduce quality.
- 1Draw the media legs
List the codec used by handset, PBX/SBC, UKDDI, recording and conference paths.
- 2Find conversion points
Confirm exactly which component transcodes and its licensed/technical capacity.
- 3Load test
Place the expected maximum concurrent calls and watch CPU, DSP channels, loss, jitter and call setup.
- 4Keep a quality baseline
Compare the same route using supported G.711 A-law so the actual saving and audible cost are understood.
Test G.729 beyond a short conversation
- Confirm G.729 is actually negotiated in SDP/call statistics.
- Test during peak utilisation, not only on an empty connection.
- Verify DTMF, IVR, voicemail, hold, transfer, recording and conferences.
- Make calls longer than five minutes and check for increasing loss or state timeout.
- Confirm SRTP and the security policy still apply.