Codec mismatch with your carrier: how to fix it

A codec mismatch happens when your server and your carrier cannot agree on an audio format for the call. The fix is to pin both ends to the same codec, almost always ulaw, with disallow=all then allow=ulaw on the carrier peer.

What a codec is and why both ends must match

A Codec is the rule for turning voice into the packets that travel over RTP. When a call sets up, your server offers a list of codecs it supports and the Carrier picks one it also supports. If the lists do not overlap, there is nothing to negotiate and the call either rejects or connects with silence.

• G.711 codec (its two flavors are ulaw in North America and alaw in much of the rest of the world) is uncompressed, high quality, and the safe default almost every carrier supports.
• G.729 codec is compressed to save bandwidth. It sounds slightly worse and historically carried a license, so not every carrier or server enables it.

How the symptoms show up

Codec problems have a couple of classic signatures:

• A 488 Not Acceptable Here rejection. The carrier is telling you it shares no codec with your offer, so the call never connects.
• One-way audio or total silence on an otherwise connected call. The call answered, but the agreed media format is not actually flowing through. This One-way audio pattern often points at codec or NAT trouble rather than the trunk being down.

The 488 is the cleaner failure to diagnose, because the carrier is telling you exactly why it rejected the call. Silence is trickier: the signaling succeeded, so everything looks fine in your logs, but no usable audio path was ever set up. Both trace back to the same root cause of two ends that never lined up on a shared format.

The negotiation, and where it breaks

Here is the decision the two sides make on every call:

flowchart TD
  A[VICIdial offers codecs] --> B{Shared codec?}
  B -->|Yes ulaw on both| C[Call connects with audio]
  B -->|No overlap| D[488 Not Acceptable]
  B -->|Different codec each side| E[Transcode]
  E --> F[Extra CPU cost]
  E --> C

Aligning the codecs

On the carrier's Account Entry, the two lines that decide everything are disallow=all and allow=ulaw. The first clears the codec list so nothing is offered by default; the second adds exactly one. That way you offer the carrier a single codec it is guaranteed to accept, and there is nothing to mis-negotiate. If your carrier runs alaw (common outside North America) use allow=alaw instead. Set the same one on both the SIP peer facing the carrier and anywhere else in the path.

After you change the codec lines, save and wait about a minute for the conf files to regenerate, then place a test call. If the 488 clears and you get clean two-way audio, the mismatch is fixed. The Account Entry is also where you set DTMF mode and host; for the full breakdown of that block see the carrier account entry guide.

Codecs are one detail in getting a carrier working. For trunks, dialplans, and routing together, read our VICIdial carrier integration guide.

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