As others have said , not likely to be your side of the connection or anything you or your provider can influence , if you have broadband and your mobile phone supports wifi calling, you could try that.
Your friend may want to check that they haven’t inadvertently added your number to a blocked list , or barred international calls or something like that , as it’s possible any message generated at the American end ( something like ‘ this line doesn’t accept anonymous calls’ ) may not be passed back to you , and all you get is a meaningless tone, I know of an occasional where someone calling a line in France was having trouble getting through, calls met with a statement read out in French , (they didn’t speak french ) , it turned out that the statement translated into English was ‘ we don’t accept anonymous calls ‘, European phone systems had to be able to interact at an operational level, there is no such compatibility with American phone systems, so it’s likely that whatever system message is being passed back by the American phone service isn’t ‘understood’ at this end, resulting in a generic ‘tone’.
