From my experience of Irish ISDN lines they either go at 64/128K or they just don't go at all. I've never seen a line's data rate decrease for technical reasons. However if your ISP is really mean they may be stuffing a load of 128K connections onto a totally under capacity point of presence.
Basically loads of people all connected to a very small pipe.
If there's a fault with your line, equipment or line card the usual symptom is that the connection will drop and voicecalls may turn to digital garble and drop (sounds like white noise)
