Hard to say what that might be. Seems like an inspection of any special settings in the GV online web application "Groups & Circles" area would turn something up. As you probably know there is a special relationship between Sprint and GV, but unless your sales mgr. has some GV setup of his own, it shouldn't be a factor (in an ideal world, anyway). I'll be interested to see the outcome of this.
A few things to cross off the troubleshooting list: You ported your original cell number to GV, and as a result you had to get another number for the cell itself, right? And nobody uses that number to call in to you, because nobody know it, right (for instance, the sales manager)? Also, since nobody calls in through the cell's current number, there would normally be no need for any conditional call forwarding feature to be active from that number to GV (at least this is how I would have it set up). Just checking the small stuff.
Btw, I'm curious: Why, after going through all the "trouble" to use GV, are you opting to use your carrier's voicemail instead of Google Voice's? Since all incoming calls are now hitting GV first (as your number port would dictate), it seems a little cumbersome to do it this way. You know, the main idea of GV being to manage your incoming calls and all, and here you are dividing things up, taking away one of the prime features of the service. Maybe there's something you like about your control/convenience ability in AT&T (ST) voicemail that I'm not seeing, or how it appears on your phone.