Find my answers below:
1. Are networks usually configured to obtain MSRN at call setup i.e. at call setup the HLR gets the MSRN from the VLR (as opposed to the VLR passing the MSRN at registration or location update)?
>There is no configuration as such.An MSRN is always allocated at call-setup
2. Could you kindly explain Implicit Detached Mode?
>Imsi Detached Mode is a case of Early Call Forwarding, where the HLR has the knowledge that the subscriber is switched of his mobile and is not active in any VLR.
Do you mean to say that regardless of the ms status given in the response from the HLR to the GMSC in reponse to routing query the GMSC routes the call the the responsible VMSC unless the HLR indicates the ms is swithced off or Implicitly Detached.
>Yes Scot is correct with his analysis. The MSRN is only used for routing the call to the VLR.