we used to get loads of calls, used the telephone pref service and have had none for a couple of months. It shouldn't matter where the call centre is based. If they are representing a company in the UK then the TPS will be effective.
http://www.tpsonline.org.uk/tpsr/html/default.asp
you can also block, junk faxes, junk mail and baby junk mail (whatever that is)
I think there was a Jetblast thread on a similar topic recently.