Receiving 603 declined instead of 486 busy


I’m testing Asterisk FreePBX 11.2.1 in a lab. My test case is called party receives 486 busy. I busy out pbx phone 1 by nailing up a call to pbx phone 2, and disable call waiting. I place a call from the pstn to pbx1 expecting 486, but I receive 603 instead. Any ideas on where to fix this in the GUI? Thanks!