An MSISDN is a mobile phone number in full international form (country code plus subscriber number), the identifier carriers use to route an SMS to a handset.
MSISDN stands for Mobile Station International Subscriber Directory Number, which is the technical name for a mobile number as the network knows it: country code, then the national number, without leading zeros or the plus sign. It is distinct from the SIM's IMSI, which identifies the subscription rather than the number.
Gateways expect MSISDNs in E.164 format. A number written the local way, with a leading zero or a trunk prefix, has to be normalised first, which is what an importer does when it knows the default country.
At SMSMeteor: every imported number is normalised to E.164; rows that cannot be resolved are listed in the import report and never billed.