Have you tried calling them back and asking them who they are ? want me to do it ?Got alot of free time on my hands at the moment either that or just leav your phone on auto answer soon they will start to get a bill i did that but you have to leave your phone somewhere quiet e.g where they cant here what your saying incase your busy forget and they ring normally i just lft it in a empty room when i didnt have time to keep an eye on it ok i had to return all the other missed calls but the stupid calls have now stopped.Also if you have a t610 ( i dont know about others) you can set it to accept calls from list only function i used that as well works a treat anyone thats numbers is not on your list gets diverted straight away to a busy tone.