Definitely not a problem - with pay as you go packages Tiscali (and anyone else for that matter) don't care which number you call from except...
..that it may have to have CLI enabled (Caller Line Identification, I think) - i.e. the phone number needs to be transmitted to the ISP, or in other words not withheld - you can check that easily by phoning a mobile from the line in question and see if the number apears on the display. This is a requirement that BT Openworld have + others I have come across so I guess it's the same with Tiscali.
hth