Synonym for "call bulls--t"

This may be inadequate for your purposes, but to "call foul" is equivalent.

My vote would go to "I call shenanigans," a fairly recent expression that may very well have its origin in euphemism, as Malvolio suggests in an answer to the question What's the origin of the common phrase "I call shenanigans"? —asked on this site back in April 2011.

The two-word phrase "call shenanigans" yields no matches in a Google Books search through the year 2008, but a standard exact-phrase search for "I call shenanigans" on Google produces more than 1.8 million matches, so it's clearly a popular phrase.