I would be very surprised if your VOIP datastream would be throttled. A byte count on the router, how pragmatic it would seem, would also not be very reliable. Once the router is rebooted the count will disappear (reset) and locking a byte count to stay intact is close to impossible without extra hardware on the motherboard. So there would be some more practical objections towards really using this, because some settings need a reboot on the router. This ofcourse will also apply to a DD WRT-ed router but not in case you just add it for this sole purpose because it won't need a reboot. So if you want accurate/reliable data (which you wanted, looking at the objections towards packet counting)
But you can always send a feature request, asking Dlink to change packet count into byte count. Which ofcourse will also be reset once rebooted. And that cannot be changed, not even with other brands.
I don't want to spoil your fun, but I am trying to give you a realistic feedback on your requirement, since you seem to like blaming the manufacturer for things that are not realistic in the first place
