My winmodem now dials out but I still can't get a full connection

I am trying to connect to the Internet using a GVC ISA linmodem with a Lucent 1643 chip. I have installed the ltmodem 8.22 driver and can get both halves of it (lt_modem.o and lt_serial.o) to load without error
provided that:
1) I use isapnp to initialise the card to the same values as Windows uses (IRQ 4, base address 0x108, com address 0x3f8, dma 7 & 5)
2) I enter the IRQ, base address and com address as a "Forced" parameter when loading lt_modem (without this, lt_serial does not detect the modem.)

I can now dial out (Hurrah!) and hear all the normal dialling sounds. But I still can't make a connection. Here is part of the wvdial output with comments in {}:
--> Modem initialized.
--> Sending: ATDT08456042086 {number for UKLINUX}
--> Waiting for carrier.
ATDT08456042086
{dialtone heard}
{dialing sounds heard}
{handshake noises heard}
{Sound cuts out - successful carrier detect?}
[30 second wait]
--> Timed out while dialing. Trying again.
--> Sending: ATDT08456042086
{From this point on, no dialling noises and no waiting}
--> Waiting for carrier.
--> Timed out while dialing. Trying again.
{repeated ad nauseam}

This is the wvdial.conf that I use:

[Dialer Defaults]
Modem = /dev/ttyLT0
Baud = 115200
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
ISDN = 0
Modem Type = Analog Modem
Phone = 08456042086 [number for UKLINUX.net]
Username = [my user name]
Password = [my password]
Stupid Mode = yes

The UKLINUX site doesn't suggest that I need to do anything more esoteric than this.

I have tried an alternative method using the ppp-on script (which starts pppd and then uses chat to connect). I then find these lines in /var/log/messages:

Aug 22 14:47:07 Aptiva chat[2454]: send (ATDT0845-6042086^M)
Aug 22 14:47:08 Aptiva chat[2454]: expect (CONNECT)
Aug 22 14:47:08 Aptiva chat[2454]: ^M
Aug 22 14:47:38 Aptiva chat[2454]: alarm
Aug 22 14:47:38 Aptiva chat[2454]: Failed
Aug 22 14:47:38 Aptiva pppd[2453]: Connect script failed
Aug 22 14:47:39 Aptiva pppd[2453]: Exit.

When I dial directly using minicom, I get the message CONNECT noEC, which suggests that I *am* finally getting some kind of connection, but then Minicom goes dead and I can't get any further. And when I check /proc/interrupts, it shows that IRQ 4 is assigned to a serial port but no actual interrupt calls seem to have been made on it (which I assume means no data is passing through the port in either direction).

What should I try next?

Further to this:

I have now found out what caused the problem after much poring over the Serial-HOWTO. For some reason, rts-cts flow control had been disabled in the modem's port. I had to use "stty /dev/modem crtscts" to enable it manually. I was then able to get a full connection.

So if anyone else has this problem - modem dials out with normal sounds but then you get timed out on the connection - use stty to check whether hardware flow control is on.

What wonderful things HOWTOs are!