Is your game crashing or are you dropping your connection with your friend? If you are dropping your connection with your friend, make sure that you both have "Call Waiting" disabled. How you disable it is by putting a certain number in the number you dial to your ISP. Most are *70 or *70, but check with your ISP or phone company to find out.

If your game is crashing, then that is another issue. But since you said a "Modem to modem problem", I am assuming you are just dropping a connection with your friend.