Phone calls

Without purchasing another device, such as a call blocker, Cox offers Selective Call Features to either Accept Only or Block Only calls.

Research at http://ww2.cox.com/residential/support/phone/article.cox?articleId=e7731350-641a-11df-ccef-000000000000

However, Selective Call Features isn't free.  Cox doesn't post their charge for this service, but I'm sure a separate call blocking device would pay for itself in a year.

 800notes has a good forum on call blockers at http://800notes.com/forum/ta-5d4e343675a5629/call-blocking-devices

Though we can't endorse a specific device or an external link, Bruce pointed to the right page on cox.com. 

Pricing for selective call features depends on package rates, and in some areas certain features may be available a la carte. Your local sales line can best assist you with details.