The issue could quite possibly be with them, keep persisting and you will get through to them. Also it could potentially be your phone socket i remember experiencing a similar issue when i was with o2Broadband some years ago for me the upstairs socket was the issue. That too was causing interference/noise through the telephone.
1.7MBPS would be about right for a max speed for something that's upto 2.5MBPS. Speed is all dependent on multiple factors, Line quality/stability, how far away from the phone exchange you are, the plan you are using they may throttle you(Not around your neck obviously), other people in the area using similar services. So on and so on.