I used to play UT in a clan on dial-up...
My main problem was when we had to play on the other clan's server. I always had a good playable ping when we played "home". But when you go "visitor" you could be connecting to a server who knows where. Least when you aren't playing clan games you can hunt and peck the good ping servers... know what I mean?
Also ONS usually doesn't tend to play well competitive wise on a dial-up due to the higher amount of players usually involved.
But hey... depending on how good your modem/phone lines are you may get away with it.
I don't wanna discourage you from clans because they really can be an awesome experience if you get into one with cool people, not a bunch of point-whores.