This is truly amusing!
Lutherans have typically argued that the universal call of the gospel cannot be a sincere one for the Reformed (this argument is present in the Formula of Concord). Why? Because it is falls on the ears those whom God predestines to damn in hell. Calvin was therefore quite explicit that the presence of the Word could not actually be a gage of election because not everyone who hears the Word is saved.
What I found reading Heppe is interesting. The later Reformed scholastics felt the force of this criticism. Hence they argued that the universality of call was in fact sincere despite the fact that God secretly intended certain people's damnation. How did they work that one? Their claim is that the call is sincere insofar as it is truthful. In other words, God says to everyone "if you repent and believe, then you will be saved." Now that's true and sincere. Nevertheless, God only works repentance and faith in the elect and so only they meet these conditions.
What's more problematic about this claim (beyond it's pure sophistry) is that it basically makes faith into a kind of condition or law, rather than a receptive organ. In fact the Reformed generally like the idea that the gospel is a sort bilateral covenant. Calvin and Zwingli both agreed that the word of the gospel was not only a promise, but a command to repent and believe. Though one does not of course merit salvation through doing these actions, the emphasis definitively falls on the obedience and activity of the human subject rather than the passivity.