This looks to be false. According to the FTC.
http://www.ftc.gov/opa/2007/02/dnccellphones.shtm
Also, it is list here as Netlore...
http://urbanlegends.about.com/od/bus..._directory.htm
So while it doesn't hurt to do this, there is not deadline, and telemarketers are already prohibitied from calling cellphone numbers with automatic dialers.