Elastic SIP Trunking Codecs

Twilio supports PCMU and PCMA for Elastic SIP Trunking.

When Twilio directs incoming traffic from the PSTN to your communications infrastructure, it will send PCMU and PCMA in the following order in the SDP parameter.

1. PCMU
2. PCMA

When Twilio receives outgoing traffic from your communications infrastructure to the PSTN, we will match the first supported codec in the SDP offer. Twilio supports PCMU and PCMA.

In addition to the default PCMU and PCMA codecs, Twilio offers limited availability to the G.729, OPUS, and AMR-NB codecs. To access these Limited Availability codecs, you must work with your account team.

Twilio AMR-NB supports Voice Activity Detection (VAD), Discontinuous Transmission (DTX), and Silence Insertion Descriptor (SID) frames.

Either party in a call can request a change of mode via a Codec Mode Request (CMR). Twilio will respond to CMRs received from call peers, but does not send CMRs.

The mode-set and octet-align media format SDP parameters are supported by Twilio. Any other AMR-NB SDP parameters will operate with their default values.

• The AMR-NB codec supports eight encoding modes, which vary in bitrate from 4.75 kb/s to 12.2 kb/s. Twilio supports all 8 modes.
• A call can be limited to a subset of these modes via the a=fmtp:<fmt> mode-set SDP media format parameter (termination calls only).
• For origination calls, Twilio does not offer the mode-set parameter. Therefore, the default set of all eight modes is offered for outbound calls.

• The payload format for a session can be in bandwidth-efficient or octet-aligned format.

• This is configurable in the a=fmtp:<fmt> octet-align SDP media format parameter for termination calls with Twilio.

  • octet-align=0 is the default, bandwith-efficient format
  • octet-align=1 is the octet-aligned format

• For origination calls, Twilio only offers the default, bandwith-efficient payload format (octet-align=0)

If you have Limited Availability codecs enabled on your account, Twilio will send the codecs in the following order of priority in the SDP message:

1. OPUS
2. PCMU
3. PCMA
4. G.729
5. AMR-NB

• PCMU will always immediately precede PCMA.
• If enabled on your account, OPUS will always be first, and will be immediately followed by PCMU, then PCMA.
• If enabled on your account, G.729 will always immediately follow PCMA.
• If enabled on your account, AMR-NB will always be last in priority.

• Twilio only offers the default, bandwith-efficient payload format (octet-align=0)
• Twilio does not offer the mode-set parameter. Therefore, the default set of all eight modes is offered for outbound calls.

If you have Limited Availability codecs enabled on your account, Twilio will match the first supported codec across PCMU, PCMA, and the additional codecs you have enabled on your Twilio account.

• Twilio supports the mode-set SDP media format parameter when receiving SDP offers.

• Twilio accepts both AMR-NB payload formats for the SDP media format parameter:

  • a=fmtp:<fmt> octet-align=0 (bandwith-efficient)
  • a=fmtp:<fmt> octet-align=1 (octet-aligned)