No, it will cancel voicemail altogether. The phone will ring indefinitely, or eventually disconnect.

To return to AT&T VM, you'll need the phonenumber for that, which should be possible to obtain with #002#, then re-set with 002*(AT&T VM number)#

I'm not sure if I'll be trying that. ;)

Actually, it did reset to AT&T. I tried a few of these playing around.