External modem connects but won't surf
 

Using a external dialup modem, I can't get beyond the ISP, and thus can't get any web pages.

I'm new to Linux. Recently, I've started moving my PCs over to Ubuntu, and things have gone very well, apart from modems. I'm trying to set up two Ubuntu PCs for a friend, and a retired aunt. They don't have broadband, so I need to get a modem working. After two weeks of trying, I eventually realised that WinModems were a bad idea, so I dug out my old external US Robotics 56k Voice FaxModem.

Ubuntu detects the modem with no problem:
finding /dev/ttyS1.
On activating the modem, it dials and establishes a connection, which it holds.

Netstat gives me an IP address from the ISP I'm dialling.
Traceroute gives me two IP addresses, one local (0.33ms) and one remote (249ms), both of which I can ping.

But that's as far as I can get. I can't get web pages, or ping other addresses, by numbers or domain. I've tried explicitly setting the DNSs to server-provided, or explicitly to those of the ISP. I've tried every suggestion I can find on the newsgroups, and I'm a bit stuck. Without internet access from Linux, my aunt and friend will have to go back to Windows! What else can I try? I must be missing something obvious.
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System details as requested:
Ubuntu Linux 5.10 (Breezy Badger)
uname -r ==> 2.6.12-10-386

/bin/lspci ==>
0000:00:00.0 Host bridge: Intel Corp. 82845 845 (Brookdale) Chipset Host Bridge (rev 03)
0000:00:01.0 PCI bridge: Intel Corp. 82845 845 (Brookdale) Chipset AGP Bridge (rev 03)
0000:00:1e.0 PCI bridge: Intel Corp. 82801 PCI Bridge (rev 12)
0000:00:1f.0 ISA bridge: Intel Corp. 82801BA ISA Bridge (LPC) (rev 12)
0000:00:1f.1 IDE interface: Intel Corp. 82801BA IDE U100 (rev 12)
0000:00:1f.2 USB Controller: Intel Corp. 82801BA/BAM USB (Hub #1) (rev 12)
0000:00:1f.3 SMBus: Intel Corp. 82801BA/BAM SMBus (rev 12)
0000:00:1f.4 USB Controller: Intel Corp. 82801BA/BAM USB (Hub #2) (rev 12)
0000:00:1f.5 Multimedia audio controller: Intel Corp. 82801BA/BAM AC'97 Audio (rev 12)
0000:01:00.0 VGA compatible controller: nVidia Corporation NV6 [Vanta/Vanta LT] (rev 15)
0000:02:08.0 Ethernet controller: Intel Corp. 82801BA/BAM/CA/CAM Ethernet Controller (rev 03)

Have you tried adding your ISP IP numbers into /etc/resolv.conf file? That may be holding it up.

I've tried KPPP for configuring the connection, and that seems to add the dynamic DNS from the ISP to the file /etc/resolv.conf.

Are there any other files that need to be set manually, that KPPP or simlar programs don't touch?