SP parameter cannot return more than 256 chars

Freetds has a bug or a limitation 
I execute this query on tsql:
"declare @iplist varchar(512),@z int exec tempdb.dbo.sip_redirect 44,
@iplist out, @z out select @iplist,@z"
 but I only get back the first 256 characters on the variable @iplist, the
rest is ignored, but I need the full 512 or more characters. How can we fix
this limitation?
I use the latest stable version. Parameter passing should be limited I guess
to 64 K if possible.