I took a look at the Help file for MediaRing Talk which came with my SB (PCI 128)
card. Here's something I found:
Force half-duplex operation
Check this box to force MediaRing Talk to work in half duplex mode. This option is
only available if you have a full duplex card. MediaRing Talk automatically detects
the duplex mode of your sound card as well as the other party?s sound card and switches
to half duplex mode if any one party is using a half-duplex sound card.
This indicates that at least some software automatically runs your SB at full duplex
if it's capable of full duplex.