Hello Lum,
you should provide more details to get better help:
a) platform : what model is your router ?
b) IOS version
c) configuration of dialer, isdn and DDR
In general, in some specific rare cases I saw the need to disable CEF or fast switching usually with IPSec and NAT, but be aware that this has an impact on router performance.
What happens if you disable CEF and fast switching only on the dialer ?
Hope to help
Giuseppe