Best way to do it is to do a transfer. Contact Tiscali and ask them for the MAC number.
Once you have it, contact Wannado and tell them you want to transfer to them from Tiscali. They will ask for the Tiscali MAC Number. Give it to them and they'll do the transfer with no new activation charge (I think) and you should not be off line for more than a few hours at most.
ADSL modems cannot be used for dial up.
Continue to use your existing modem, do not use the Wannado provided USB modem. They will give you the new settings to put into it.