This setup is known to work on RH5.1 and RH5.2.
Start with these diald rpms. Older ones had problems with dctrl.
diald-0.16.5a-1.i386.rpm
diald-config-0.16.5a-1.i386.rpm
These were uploaded to ftp.redhat.com/contrib by Duncan Haldane.
They are now on ftp://contrib.redhat.com/libc6/i3
Diald setup can be easy if you follow Jeff Balderson's
proven method, attached.
Here are two more tips that might be useful:
1. Enter your primary nameserver 2 or 3 times in /etc/resolv.conf.
This increases the domain request timeout, letting domain requests
succeed while the modem is connecti