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.
Mobile Subscriber ISDN numbers (i.e. E.164 numbers) are assigned from the E.164 numbering plan 
; see also ITU-T Recommendation E.213 
. The structure of the MSISDN will then be as shown in Figure 2.
The number consists of:
Country Code (CC) of the country in which the MS is registered, followed by:
National (significant) number, which consists of:
National Destination Code (NDC) and
Subscriber Number (SN).
For GSM/UMTS applications, a National Destination Code is allocated to each PLMN. In some countries more than one NDC may be required for each PLMN/mobile number ranges.
The composition of the MSISDN should be such that it can be used as a global title address in the Signalling Connection Control Part (SCCP) for routeing messages to the home location register of the MS. The country code (CC) and the national destination code (NDC) will provide such routeing information. If further routeing information is required, it should be contained in the first few digits of the subscriber number (SN).
A sub-address may be appended to an E.164 number for use in call setup and in supplementary service operations where an E.164 number is required (see ITU-T Recommendations E.164, clause Annex B, B.3.3, and X.213 annex A). The sub-address is transferred to the terminal equipment denoted by the ISDN number.
The maximum length of a sub-address is 20 octets, including one octet to identify the coding scheme for the sub-address (see ITU-T Recommendation X.213, annex A). All coding schemes described in ITU-T Recommendation X.213, annex A are supported in 3GPP networks
As an exception to the rules above, the MSISDN shall take the dummy MSISDN value composed of 15 digits set to 0 (encoded as an international E.164 number) when the MSISDN is not available in messages in which the presence of the MSISDN parameter is required for backward compatibility reason. See the relevant stage 3 specifications.