How far through the connection does it appear to be getting? Since it's an external check that you have "OH" and "CD" lighting up, also you should see RD and SD blink a few times. IF it doesn't hang up right away (CD/OH going off again) then you're probably logging in OK and the next thing to look at is routing or DNS. If things don't light up, you need to make sure that the modem device is definately /dev/ttyS0 (usually Redhat will make a link /dev/modem which is a shortcut to /dev/ttyS0 and use that for all the settings) /var/log/messages and /var/log/pppd should contain more clues.
The other option is bring it along to a meeting.
We can check that the modem is being talked to correctly, but we do not have a venue that allows us to actually use a phoneline to test dialup, so please make sure you give us some advanced warning if you plan to come to a meeting just to get dialup issues fixed. Cheers Jamie