Depends on how you want the finish to look when you're done with the call.
For a duck call a couple really good options are dipping in spar urethane (thinned with mineral spirits) or doing a CA finish. Both will be glossy. Another option is Tru-Oil (multiple coats), but I don't care for it on my duck calls.
For an oil-finished look you could use Velvit oil.
For a beginner the spar dip is pretty hard to beat. You'll want to touch up the toneboard's raw wood with Velvit oil to protect it.
