Let me try and stop the confusion: our friend Peyre just tries to say that if it doesn't even work without DOSBox, then there's no way it would with with DOSBox.
Then again, Peyre's screenshot didn't tell us which port "does not exist".
But more importantly, Peyre is the only one here that talks about an actual modem. The rest of the posters talk about DOSBox's infamous fake modem which phone number is actually an IP address (i.e. it turns old games' modem support into TCP/IP [read: Internet] support).
...assuming DOSBox v0.63 does work with dotless IP addresses, as that is what started this whole discussion in the first place!
So Peyre, if it's a fake modem you want, just try running the game through DOSBox and enter your friend's IP address instead of his phone number. Of course, this is assuming he runs the game through DOSBox too and both of your firewalls accept outcoming/incoming connections (the latter at least for the host) from the port that is mentioned in your two DOSBox's configs!
However, if it's a real modem you're after, that's a whole other setting in DOSBox's config. But nothing would work until your modem works successfully in Windows natively! Start by entering its properties and check for its port!
Hope I cleared up the mess...