The 2.4.x series of kernels. The kernel and module support has changed significantly for the 2.4.x series of Linux kernels, and require a number of additional actions on your part. The older 2.3.x series of ppp will not work with the new kernel. You now need to set up the ppp options so that you can get debugging information when you try to connect, and so that it is minimally usable. In general do not do so unless advised to do so.
More harm is done by putting in too many un-understood options than by putting in too few.
Bill Unruh works for the Advanced Research Department of the Canadian Institute for Physics and Astronomy.