How to block international numbers using Cox Call Screen (*60)?

I have Cox call screen service and I love being able to block people who bother me but quite a few calls I get are international. When I try to add the last number dialed to the list it tells me it is unable to do so. Do I have to enter it in manually? If so, how? Do I diall 011 then the country code? Or how? Is there anyway to block these types of calls?
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