VOIP worked just fine with OFP when you ran server in directplay mode instead of sockets. Problem was quality, your voice was too robotic.
As for ArmA, there seems to be no option to run it in directplay mode? Atleast not for self hosted non-dedicated servers. VOIP worked just fine with v1.03 demo, but not with v1.05 full version. This is just my experience with VOIP.