Normal Codec Selection

Codec selection is based on the Offer/Answer model specified in RFC 3264.

1. The calling endpoint issues an offer that includes a list of the codecs it supports.

   • For IP Office SIP trunks, the IP Office offers the codecs set on the SIP trunks VoIP tab. It does not offer those set on the extension.

2. The called endpoint sends an answer that normally contains a single codec from the offered list.

   • If there are multiple codecs in the answer, IP Office only considers the first codec. If the SIP Line is configured to do Codec Lockdown, it will send another INVITE with the single chosen codec.

Codec Changes with reINVITE

For R11.0 and higher, the IP Office supports codec selection following a reINVITE. Previously, when a reINVITE was received during a call, if the reINVITE contained the codec currently in use, that codec was preferred and kept. For R11.0 and higher, the IP Office reevaluates the codec to use based on any preferences included in the reINVITE:

• For example, if the endpoint/trunk has a different codec preference to the system, hold/unhold sequences will result in codec changes. When held, the system codec preference is used to play music-on-hold. When unheld, the codec preferences are reevaluated.

When using this behavior:

• Direct media is supported for SRTP phones that change keys on each reINVITE.

• The IP Office supports the transfer of video calls.

Note:

• The new behavior also applies to SM lines and SIP extensions.

• On IP Office systems upgraded to R11.0 and higher, SLIC_PREFER_EXISTING_CODEC is automatically added to the SIP Engineering tab of any existing SIP lines to retain the existing pre-R11.0 behavior.