Informix has no product called SQLPLUS.
You need to be more specific. Which Informix products and
which versions are you using ?
I think you talk about 'isql'.
AFAIK 'isql' does not support
the calling of stored procedures over the interactive
SQL interface.
You can use 'dbaccess'. This tool is delivered with the
databaseserver and supports the 'execute procedure..."
statement over the interactive SQL interface.