What the Allowed Codecs field does on a VICIdial phone

Every VICIdial phone entry has an Allowed Codecs field. A codec is the rule for how voice audio is compressed before it travels over the network and decompressed at the other end. This field lets you pin down exactly which ones a given phone is permitted to use.

If you are not sure what a Codec is or how it interacts with the rest of a phone record, this field is a small but real part of getting clean audio.

What the field accepts

You enter a comma-delimited list of codec names, no spaces required. Common values include ulaw, alaw, gsm, g729, speex, g722, g723, g726, ilbc, opus, slin and a few others. The order you list them in is the preference order the phone will offer.

• ulaw and alaw are uncompressed and sound clean on a local network.
• gsm and g729 compress harder to save bandwidth, which matters for remote or low-bandwidth agents.
• Some codecs may not be installed on your box. g729, g726 and opus are the usual ones that are missing unless added.

What happens when you leave it empty

Empty is the default, and empty is usually fine. When the field is blank, the phone falls back to the system default codecs, or to the codecs from the phone entry directly above this one in the list. So you only set this when one specific phone needs different handling from the rest.

There is a companion option, Allowed Codecs With Template. Setting it to 1 forces your codec list to be included even when a conf file template is used. Default is 0.

How the codec list is chosen

flowchart TD
  A[Phone places or answers a call] --> B{Allowed Codecs set}
  B -- Yes --> C[Use the listed codecs in order]
  B -- No --> D[Use phone entry above this one]
  D --> E{Still empty}
  E -- Yes --> F[Use system default codecs]
  C --> G[Negotiate with Asterisk]
  F --> G
  G --> H[Audio path established]

This negotiation happens between the device and Asterisk for both SIP (Session Initiation Protocol) and IAX2 phones. The phone offers its allowed list, Asterisk picks the first one both sides support, and that becomes the call's audio path.

For the full picture of how a phone record is built, see the VICIdial phones and aliases guide. If you are deciding between protocols first, read SIP vs IAX2 phones.

Codecs are the kind of low-level detail you should not have to babysit. VICIfast runs the dialer and tunes these defaults for you, so see pricing.