The only two calls I am aware of for squirrels are the bellows (shown above) and a squeal type call (you blow into it). The bellows sounds like they are chatting (supposed to be calling others for feeding) and the squeal imitates a squirrel in distress from a hawk, etc. I never had much luck with the squeal call.

You could also rub two quarters together on the rough edges or take two hickory nuts an hit them together. They will sometimes work.

I killed three fox squirrels last week with the white muzzleloader shotgun, all were on the ground. I put the sneak on them to get in range and shot them up in tall oak/maple trees.