Function answer_replacing_with 

pub async fn answer_replacing_with(
    endpoint: &Handle,
    incoming: &Incoming,
    media_address: IpAddr,
    replaced: &mut Call,
    codecs: Codecs,
) -> Result<Call>

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

codecs applies to the replacement, which is the only call being negotiated here. The one being replaced is hung up, and nothing renegotiates it on the way out.