As far as know, the difference between N and ND is that the 1043N has the undetachable antennas and 1043ND has detachable antennas, And other hardware design should be the same.

you can also contact the tp-link for professional advice

Are you in China? maybe you can find some Chinese friend to help you. Or contact the tplink Chinese technical support

You can refer to http://www.tp-link.com/en/article/?faqid=400 for troubleshooting

do you know that the VOIP utilizes the H.323 technology or SIP technology?

I seem to have the same issue before, but the router started working normally again after a while. Have you tried to connect your computer directly to modem? Do you have a cable modem or DSL modem?...