SMS FAQ: Country Restrictions

What countries do you support for SMS service?+
What are limitations and restrictions on US Long Code SMS?+
What kind of restrictions are applied on Sender-IDs in different countries?+
Which countries do not mandate pre-registration of Sender-ID?+
Which countries require pre-registration of Sender-ID?+
In which countries is the Sender-ID pre-registration recommended but not mandatory?+
Which countries need a Long Code phone number?+