Metin UZAR

All these refer to ISDN being Integrated Services Digital Network, however MSISDN is a directory number and has nothing to do with ISDN. It is defined as:
Mobile Station International Subscriber Directory Number (MSISDN)[citation needed] – a number used to identify a mobile phone number internationally. MSISDN is defined by the E.164 numbering plan. This number includes a country code and a National Destination Code which identifies the subscriber’s operator.
E. 164 states – The international public telecommunication numbering plan. An E.164 number uniquely identifies a public network termination point and typically consists of three fields, CC (country code), NDC (national destination code), and SN (subscriber number), up to 15 digits in total.

Source Wikiepedia
MSISDN Format

An MSISDN is limited to 15 digits, prefixes not included (e.g., 00 prefixes an international MSISDN when dialing from Sweden).
MSISDN – Mobile Station International Subscriber Directory Number
In GSM and its variant DCS 1800, MSISDN is built up as
MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code, identifies one or part of a PLMN
SN = Subscriber Number
In the GSM variant PCS 1900, MSISDN is built up as
MSISDN = CC + NPA + SN
CC = Country Code
NPA = Number Planning Area
SN = Subscriber Number
Example
MSISDN: 79261234567
CC 7 Russia
NDC 926 MegaFon
SN 1234567 Subscriber’s number
For further information on the MSISDN format, see the ITU-T specification E.164.