What you are trying to do is not an instance method. Instead of calling only User.CallUserFunction()you need to do something like this:
User model
module.exports = {
attributes: {
name: {
type: 'string',
required: true
}
},
CallUserFunction: function(){
}
}
User controller
module.exports = {
create: function(req, res){
sails.models.user.CallUserFunction();
}
}
edit: forgive any typo, since I'm not in front of my dev workstation, to check this out, I really know her very close to this syntax sails.models.