09.123GPPApplication of ISDN User Part (ISUP) Version 2 for the Integrated Services Digital Network (ISDN) - Public Land Mobile Network (PLMN) signalling interfacePart 1: Protocol specificationTS
The Message Transfer Part (MTP) protocol shall conform to the requirements in ETS 300 008  with the appropriate value for the Network Indicator (NI).
It is preferred that NI value 10 (national network) is used within GSM networks as in the fixed national network. Based on national agreement, any NI value may be applied on the interface between GSM networks and the fixed networks.
Annex A (informative): Considerations on teleservices