The MAP messages affected by MNP are the MAP_SEND_ROUTING_INFORMATION (SRI) message without OR parameter set sent to the HLR and the MAP_ANY_TIME_INTERROGATION (ATI) message with parameter MNP-requestedInfo present terminated in the MNP-SRF. In a PLMN supporting MNP with direct routeing using signalling relay, all incoming calls and calls originated in the network for which the called party number is within the ranges owned by any network in the portability domain, the gateway MSCs will send an SRI such that it will be handled by the MNP-SRF in that network. In a PLMN supporting MNP with direct routeing, where the called party number is within the ranges owned by any network in the portability domain, the INE sends an MNP Information request (ATI) such that it will be handled by the MNP-SRF in the network. In a PLMN supporting MNP with indirect routeing using signalling relay, all incoming calls and calls originating in the network, for which the called party number is within the range owned by the network, the gateway MSCs will send SRI such that it will be handled by the MNP-SRF in that network. In a PLMN supporting MNP with indirect routeing, the INE sends an MNP Information Request (ATI) such that it will be handled by the MNP-SRF in the Number Range Holder Network. The MNP-SRF obtains routeing information from the NP database to identify the subscription network associated with a particular national MSISDN. The interface between the MNP-SRF and the NP database is considered implementation dependent and is not detailed further. From the perspective of the PLMN in which the MNP-SRF resides, the CdPA represents one of:

1. An own number ported out.
2. An own number not ported out.
3. A foreign number ported in.
4. A foreign number ported to a foreign network.
5. A foreign number not known to be ported.

Cases 4 and 5 are applicable only for direct routeing. For SRI: In case 1, the MNP-SRF may perform one of the following depending on agreements within the number portability domain.

1. An SRI response is sent containing the necessary routeing information to route the call to the subscription network. This is performed by an internal MAP Application Termination Function (MATF) known as the Number Portability Location Register (NPLR).
2. If indirect routeing of calls with reference to the subscription network is used, the message is relayed to the MNP-SRF in the subscription network, whose NPLR provides the necessary routeing information in an SRI response. The use of an NPLR in the subscription network can only be by agreement within the number portability domain.

In cases 2 and 3, the MNP-SRF relays the message to the HLR. For further details of the signalling relay function, the reader is referred to [7]. In case 4, an SRI response is sent, containing the necessary routeing information to route the call to the subscription network. In case 5, an SRI response is sent, containing the necessary routeing information to route the call to the number range holder network. For ATI: In all cases the MNP information request (ATI) is sent to the MNP_SRF_MATF_Info_Request procedure/ MAP Application Termination Function (MATF) for handling. A response is sent to the INE containing MNP information or an error is returned.