Since all of the bigtime telemarketers, such as MCI, use T1 lines to place their calls, their callerid shows up as "Unknown Caller." Our teleco has a service that if an unknown caller calls our number, they get a recording and must press #1 to continue and they also must say their name/company before the teleco's system calls our number. When someone on our end answers, it'll ask us "Do you wish to accept a call from XXX?" If not, you just hang up. It's been over a year since we began testing that service, and well, we haven't had a single telemarketer call that line since.