SDP Manipulated by RFC 2833 Mode

If none of the telephone-event Modification by Codec Policy events occur, as previously explained, SDP may still be modified by RFC 2833 Mode parameter if preferred or dual mode is configured.

The RFC 2833 mode parameter functions similarly to the add-on-egress parameter; it suggests telephone-event support, by adding it in SDP if not already there. This parameter must be set to preferred or dual to add telephone-event to SDP.