I had similar issues, and I complained enough that my ISP gave me a second, just plain stupid modem and a splitter, so now I have their fancy phone/modem/router/wifi one just for the phone, and a simple modem only box for my actual internet. Works like a charm now!
Just keep in mind, their "first tier" of customer support probably can't authorize that kind of thing, and probably won't even know what the heck you are wanting. I complained about the crappy slow internet and bad ping until I got to their actual tech guys, and told him the older modem did better, and he was like "Well, why don't you use one of them?", I said "Need the phone", and he told me to just put a splitter on it and use both, he'd set it all up and have the local office put one aside for me. I only even pay rent on the one modem, so it's actually an amazingly good setup, other then meaning I've got the start of my own "commercial grade" networking setup attached to a wall in the basement, with the modems, the router, a switch, and a server.
