Function audio_answer 

pub fn audio_answer(
    session_id: u64,
    host: &str,
    port: u16,
    payload_type: u8,
    telephone_event_pt: Option<u8>,
) -> String

Print an SDP answer committing to one G.711 codec, optionally accepting RFC 4733 telephone events (DTMF) on the payload type the offer proposed.

session_id doubles as the o= version; pass something unique per call (a timestamp, a counter). host/port are where we will receive RTP.