dozen robocalls on landline within an hour

Depends on the service you have.  Is it traditional POTS or digital service? POTS you can block 10 numbers, digital give you 100 with additional options with digital call protect.   There's also 3rd party devices for blocking calls. 

i am pretty sure it is traditional. How do you block a number when you don't know the number there calling from.

Why don't you know the number? What does your caller id show?  If it shows unavailable or unknown?  If so that means no data was sent, so you can't block it.

don't have caller id

In that case there's not much you can do.  Any call blocker requires you know the number to block a call.  The only exception would be if you have a feature to block the last call, which you wouldn't on a POTS line. 

From the FTC Website

There are some exemptions to the Do Not Call rules. Because of the limits to FTC’s authority, the Registry does not apply to political calls or calls from non-profits and charities (but the Registry does cover telemarketers calling on behalf of charities). Also, calls from legitimate “survey” organizations are not covered because they are not offering to sell anything to consumers. Finally, calls are permitted from companies with which you have done or sought to do business. Specifically, a company can call you up to 18 months after you last did business with it.

Also the fact the Do not call list is mostly ignored by spammers.