I registered my land line when I had one, but I only use a cell now. So far I have not received telemarketing calls on my cell and for that reason I have not registered my cell number. I don't want it on a list of numbers to not call, which would then become a list of numbers TO call after a certain period of time. You really should be able to opt out for all time from telemarketing calls and junk mail.
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Never trouble trouble until trouble troubles you!