We'll likely start with just using String / VARCHAR as type for SMS phone numbers in client/staff, but in order get serious about SMS support, the "data quality" of those will quickly be fairly import.
Therefore, it would probably be very useful if the platform technically enforced that such phone numbers are always represented and stored in the database including country code prefix (using the +91 ... notation).
The should also have some validation logic in the UI enforcing this (FrontlineSMS says: "This number is not in international format. This may cause problems matching messages to contacts.").
The UI could assist in making it easier to capture phone numbers and propose a default country code saved in some System Administration configuration somewhere when entering new phone numbers.
The Java back-end could use some proper strongly typed self validating PhoneNumber value object type, instead of just passing around String? May be something like this exists already?