Actually, this type of behavior can be both, but it fits better into the category of trespass. By putting my name on the FDNC list, I've told telemarketers they are not welcome on my property, which consists of my phone and the lines I do use. If they violate my wishes, they commit trespass.
Note that any contract I signed with the phone company in no way applies to telemarketers- it just keeps me from suing the phone company. Telemarketers are not a party to my phone contract.
Nobody on this thread has stated what authority the Federal government has over a voluntary transactions such as you signing up for phone service.
That's a straw man argument- the feds are not exerting any authority over my voluntary transactions witht the phone company- my contract with them continues the same as always. Rather, they're using their Commerce Clause power to allow me, VOLUNTARILY, to sign up for a list. Who are you to say that I shouldn't have the right to sign up for the FDNC list?