Section: 3.2 Numbering plan requirements
Content: In principle, it should be possible for any subscriber of the ISDN or PSTN to call any MS in a PLMN. This implies that E.164 numbers for MSs should comply with the E.164 numbering plan in the home country of the MS. The E.164 numbers of MSs should be composed in such a way that standard ISDN/PSTN charging can be used for calls to MSs. It should be possible for each national numbering plan administrator to develop its own independent numbering/addressing plan for MSs. The numbering/addressing plan should not limit the possibility for MSs to roam among PLMNs. It should be possible to change the IMSI without changing the E.164 number assigned to an MS and vice versa. In principle, it should be possible for any subscriber of the CSPDN/PSPDN to call any MS in a PLMN. This implies that it may be necessary for an MS to have a X.121 number. In principle, it should be possible for any fixed or mobile terminal to communicate with a mobile terminal using an IP v4 address or IP v6 address.