Cisco SPA504 - call-waiting ringing to speaker instead of in ear

I have users with Cisco SPA504gs. They tell me that when their ring group rings and hits their call-waiting it rings not in their ear on the handset but to the speaker. Obviously this is painful when you are on a call already. I have never had this occur before. Anyone know what might be going on? I assume it is a phone setting but I cannot find it for the life of me! Thanks!