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When the phone is turned on, it listens for a signal. If it receives a signal, it looks at the SID carried by the signal, and compares it with the one that is stored in the phone. Originally, in analog systems, the mobile would turn on the roaming indicator if the SID was not the single value stored in the phone.
The 4 digit prefix of an IRM (known as an IRM network identifier) is allocated by IFAST that attempts to facilitate international roaming by minimizing conflicts with North American MIN codes that are based on the national 10 digit phone number. The last 6 digits of an IRM are allocated by the carrier, providing one million unique identifiers.
Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s that is used to setup and teardown telephone calls on most parts of the global public switched telephone network (PSTN).
Worldwide distribution of country calling codes. Regions are coloured by first digit. Telephone country codes, but also sometimes referred to as "country dial-in codes", or historically "international subscriber dialing" (ISD) codes in the U.K., are telephone number dialing prefixes for reaching subscribers in foreign countries or areas via international telecommunication networks.
The HLR will send to the VLR a MAP PRN (Provide Roaming Number) message to obtain the MSRN of the roaming cell phone. Like that the HLR will be able to route the call to the correct MSC. With the IMSI contained in the MAP PRN message, the VLR assigns a temporary number known as the mobile station roaming number (MSRN) to the roaming cell phone.
The mobile country code consists of three decimal digits and the mobile network code consists of two or three decimal digits (for example: MNC of 001 is not the same as MNC of 01). The first digit of the mobile country code identifies the geographic region as follows (the digits 1 and 8 are not used): 0: Test networks; 2: Europe
In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent digitally encoded directly to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound is encoded digitally as a pulse-code modulation (PCM) signal in a 64-kbit/s ...
Mobile phones use geographic area codes (two digits): after that, all numbers assigned to mobile service have nine digits, starting with 6, 7, 8 or 9 (example: 55 15 99999–9999). 90 is not possible, because collect calls start with this number. [5] xx 7: xx 8: xx 9: British Indian Ocean Territory +246: 380 7 387 British Virgin Islands +1: 284: 10