Flextel allow you to blacklist even withheld numbers, as long as they were able to see the number. 99% of the time they seem to be able to, although they will hide some of the number from the customer (e.g. the withheld number will be shown as something like 01234 567xxx). The last three digits of the number are still masked for privacy reasons.
Call rates may not necessarily be on the cheap side for some people though. You could also make use of their IVR system to force the incoming caller to press a number (such as 1) to verify that they are a real person.
Edited by Ixel (Mon 31-Aug-20 19:14:02)