The term MSISDN relates to a mobile number in international format with the "+" sign knocked off. You can find further details via http://en.wikipedia.org/wiki/MSISDN.
International format means not having any gaps, spaces, dashes or leading zeros.
Here are some examples:
USA: 12481234567 (Country Code = 1, network prefix = 248, number = 1234567)
South Africa: 27821234567
United Kingdom: 447912345678
South Korea: 821012345678
For example In the UK if you ask someone for their phone number, 99% of people will tell you its 079xxxxxxxx. This is because its how you dial the number to make a call in the UK.
The same number in its MSISDN international format that number is +4479xxxxxxxx. This is because to dial that number, the + represents what you dial for an international call, e.g. in the uk you dial 00 for an international call, in USA it is 011, e.g. 0114479xxxxxxxx.
When the same number is sent to or passed to you from txtNation's systems it will always be in MSISDN International format, e.g. 4479xxxxxxxx.
A mobile number in the UK would be 07912345678 this is the same as +447912345678 and 447912345678, but when sending an SMS to that number it has to be in the later format.
If you are collecting MSISDNs from users via a web form, it is advisable to accept either way and automatically convert it to MSISDN format for them ready for processing the SMS to them.
ALL txtNation SYSTEMS USE INTERNATIONAL MSISDN FORMAT FOR MESSAGE PROCESSING, YOU MUST PASS / ENTER THE NUMBERS IN THIS FORMAT.