ANI being overwritten by callerid on ZAP channel

We have arranged with our provider to be able to set ANI on outbound calls for 911 use. However, whenever ANI is set with Set(CALLERID(ANI)), it is always set to the callerid number on the receiving side. I have verified that CALLERID(ANI) is set with a NoOp before the dial occurs. Is this a bug in 1.4.11, an issue with the Zaptel channel driver or a setting that I am missing?