telephone-event Modification by Codec Policy

For qualifying if telephone-event modification in SDP was performed by codec policy, one of the following events had to have happened:

• Codec policy explicitly deleted telephone-event by configuring allow telephone-event:no
• Codec policy explicitly added telephone-event by configuring add-on-egress telephone-event
• Codec policy implicitly denied telephone-event by allowing one or more codecs but not adding telephone-event to the allow list
• Codec policy has audio:no configured in the allow list

The following three cases highlight how codec policies can manipulate telephone-event SDP. Once any of these cases occurs, SDP manipulation by RFC 2833 Mode parameter will not be performed.

1. telephone-event added to SDP: The codec policy adds telephone-event to the SDP sent to the egress realm. The UA supports telephone-event.
   
2. telephone-event maintained in SDP: The codec policy maintains the offered telephone-event in the SDP sent into the egress realm. Although telephone-event was not answered in the egress realm, telephone-event is added back to the offerer-side SDP because of the add-on-egress setting in the ingress realm.
   
3. telephone-event removed from SDP: codec policy removes telephone-event from the initial SDP offer. telephone-event is not forwarded to the Answerer, and subsequently not returned from the answerer, or forwarded again to the offerer.