Unfortunatly I don't know anything about modem specifics.
I work for an overseas ISP. At the moment we only offer dialup and the #1 complaint is how its *our* fault that the customers get disconnected so frequently when in actuality that's the farthest from the truth. One reason your brother connects well and you don't is that one type of modem might be more compatable with the ISP Server's modem than others. Someone help me out here...I don't mean brand names per se, but I guess there names for different types of modems - an older type and a newer (not v90 vs. 56K either)
sigh. My limited knowledge would have you look up your brother's modem in his Linux "Device Manager" area and buy that same type since you say he can stay connected w/no probs.
Sorry I can't be more specific - I'm still learning
Hopefully someone can help me expand on this issue.