Function dial 

pub async fn dial(
    endpoint: &Handle,
    target: Target,
    to: &Uri,
    options: &DialOptions,
) -> Result<Call>

Place a call, retrying once if the far end refuses the session interval.

RFC 4028 §7.3: a 422 is not a refusal of the call, it is a counter-offer of an interval. Retrying is bounded to a single attempt on purpose — a peer that answers 422 to its own stated minimum is broken, and a loop there is an outbound call flood.