In order to make or receive calls on the GSM system an MS needs to perform one procedure, an IMSI attach. The IMSI attach is shown in Figure 21, at IMSI attach a connection between the MS and the GSM network is established. The end user does not need to specify which fixed networks he wants to use because all fixed networks follow the same numbering plan (E 164), it should be noted that the MSISDN is always the same no matter which country and to whom it calls.
