Function ring_offer_early 

pub async fn ring_offer_early(
    endpoint: &Handle,
    incoming: &Incoming,
    status: u16,
    reason: &'static str,
    media_address: IpAddr,
    direction: Direction,
) -> Result<Ringing>

Ring with an SDP offer in a reliable provisional response.

The initial INVITE must be offerless. Its caller owes the answer in PRACK (RFC 3262 section 5), and Ringing::on_prack does not acknowledge the negotiation until that answer parses and establishes the early session.