Unfortunately, your return string is not quite big enough:

SELECT dbo.fnSpellInteger ( 1999999999 )

Gives me:

One Billion Nine Hundred Ninety-Nine Million Nine Hundred Ninety-Nine Thousand Nine Hundred Ninety-

The function would be better written to return VARCAHR(MAX) - or at least VARCHAR(112):

SELECT LEN(dbo.fnSpellInteger ( 1777777000 ) + ' ' + dbo.fnSpellInteger ( 777 ))

Of course, it could easily be extended to BIGINT as well 🙂