A roamer in a visited network may miss an incoming call. It may be intentional, as the roamer may choose not to answer for any of a variety of reasons; for example the roamer may be busy in a meeting or may avoid incoming calls from an unknown caller to save cost.
Figure 12-6 Short dial codes—an example of implementation.
It may also be unintentional, as the roamer may be away from the phone or the phone may be switched off. In any case, it is desirable to get caller ID, as in the case of calls in the home network. Unlike in the home network, calling line identity (CLI) does not work while the mobile phone is roaming. Many wireless service providers have implemented missed-called alerts for their outbound roamers if the call originates from the home network. These alerts are sent via an SMS containing caller ID. This can be offered as a subscription service. Charging a subscription fee and the increased calls to the home network benefit the wireless service provider. With this feature in place, roamers have knowledge of all the calls from the home network. Roamers can also use this feature to screen the incoming calls and call back to important numbers only.
Implementation
The essential elements to implement this service are:
1. Real-time monitoring of MAP provide roaming number procedures toward partner networks. This provides correlation of IMSI in the forward message with the MSRN in the response message.
2. Real-time of monitoring of ISUP IAM and REL messages to roamers. This filters all the calls to the roamer with a specific release cause code, e.g., busy or no answer.
Figure 12-7 shows one of the possible implementations. The CCS7 SCCP links carrying MAP traffic to partner networks are monitored for the following MAP procedures.
■ Update location: This will provide critical information such as roamer IMSI, MSISDN, and country and network where the roamer is currently registered.
■ Provide roaming number: This indicates that the HPLMN needs to terminate a call to its outbound roamer. The IMSI, MSISDN, and MSRN correlation can be built by using UL, PRN, and PRN response.
The system monitors the ISUP links for all the calls routed to a partner network with the same B party number that the MSRN previously captured in the PRN procedure. If the call gets completed, then no further action is required. If the call failed for reasons specified by the administrator (e.g., user busy, no answer), then a trigger is set to send an SMS informing the roamer of the CLI as extracted from the IAM message, i.e., calling party number.
Figure 12-7 Missed call alert—example implementation.
