I don't have an answer for you, but I CAN tell you that if you manage
to do this, the speed with be excrutiatingly slow.
If you want to use your existing dial-up service, you have to use the
mobile as a modem over the GSM network, which gives a maximum of 9Kbps
speed (a normal dial-up modem gives 56Kbps and a broadband connection
will give between 256Kbps and 20Mbps or 20,000Kbps).
I think a better solution would be to find a 3G/Edge/UMTS card or
modem for your laptop. In Spain Vodafone provides them for just over
€30/month and you get up to 3.6Mbps download speed). Or alternatively,
find a way of connecting the mobile to the laptop and using its Edge
capabilities for a high-speed connection - although check the rates of
such a connection with your mobile network provider first.
Roger.