Function answer_ringing_with 

pub async fn answer_ringing_with(
    endpoint: &Handle,
    incoming: &Incoming,
    media_address: IpAddr,
    ringing: &Ringing,
    codecs: Codecs,
) -> Result<Call>

answer_ringing, from a chosen codec set rather than the default one (M-30).

The selection is made here rather than at ring because ring sends a bodiless provisional: nothing about the session has been said yet when it goes out, so the answer this builds is still the first one. That is exactly what separates this from answer_early, where the answer left in the 183 and the choice had to be made with it.