500k flow ccy spot via voice? I think you are joking. It's easier for a client and a bank just to trade via an electronic platform. The voice trader would likely direct the client to his on screen rates and he would trade there. I suggest anything under 15mn would be traded via screen.
Last time I was on an FX trading floor (2010) clients seeking voice quotes for spot would get a sales person who would be using the sales version of the FX single dealer platform. Sales guy would voice quote the electronic rate made by the auto trading system that was shown in the Java SDP GUI on their desktop. For top clients the spread might be tightened slightly, but most would get the vanilla electronic rate. No human trader would be involved, and there would be no manual price discretion at any point.