I would say if your other car has a usb, see if it works there, or inside at home. It probably does, so that rules out your device. Then, what can we say, take it to Buick. I know for a fact the bluetooth is buggy, sometimes when the other party disconnects (I'm only guessing I don't want to troubleshoot for buick) and say the disconnect supervision is such that the phone itself takes x ms to hang up, the Buick never sees the hang up and the whole radio/bluetooth is hung with a handset icon on the display. Now, fan is slow, no radio is possible, etc. I don't know how many years Buick has offered this system, but if it's the 5th model year they shouldn't still have bugs. But what are you and I gonna do, they sell 3k cars/mo. that's 36k per year, not enough to put a lot of R&D into the issues I guess.
And I don't blame you for using the USB, it provides the best sound quality possible in the vehicle. But couldn't they have designed it so that an iPod actually could fit with the door closed?
Bluetooth is constantly evolving and changing. There are several different versions of it out there. The problem could be your phone not sticking to the standard as well, depending on the version it is using and if it is doing anything backward compatible for other versions. It isn't just simple to say it is buggy because it doesn't work for you. I for one, don't have this problem.
Also, I am pretty sure that if you hung up the phone using the button on the steering wheel when you were in this situation, the problem would be gone.
But, speaking of disconnect supervision, did the other side have disconnect supervision. Do you see the call on the phone still? I have seen some phones systems that never send a disconnect signal and you have to hang up your end.