22.0793GPPService definitionStage 1Support of Optimal Routeing (SOR)TS
5.3.1 Non-support of SOR in the VPLMN of the caller
If subscriber A roams into a network not supporting OR, she may not benefit from OR and only scenario 1 and scenario 2 of SOR are supported.
5.3.2 Non-support of SOR in the HPLMN of the B subscriber
If the HPLMN of the called party does not support SOR, SOR cannot be invoked and the route of the call as far as the called party’s VPLMN cannot be optimised (the call would normally be routed via the HPLMN of the called party).
The same procedure shall be followed if the HPLMN operator denies OR on a subscriber basis.
5.3.3 Non support of SOR in the VPLMN of the B subscriber
If a subscriber whose HPLMN supports SOR registers in a VPLMN which does not support SOR, only the cases of Early Forwarded calls may be optimised.
5.3.4 Calls to special mobile network numbers
If the called number is a special mobile network number of the HPLMN, the HPLMN has to guarantee that the call terminates at its correct destination. This can be achieved by denying OR for this call by sending an appropriate error cause or by implementing a special handling.