We have done this 2 ways. One ways involves buying, if you do not already own, "softphone" licenses from 3Com (why they make you pay for this, I do not know. Every other vendor gives it for free). Using the softphone interface, you are setting up the remote computer with a software-based 3Com phone that would connect to the internal IP of your NBX. We have done this over a cable and dsl connection and it works, although it is quirky.
The other way which has been more successful for us is using a 3rd party product called Crystal Voice. The downfall of this option is you need to setup a seperate box to run the CV server. You install a software client on the remote pc and attach a handset to the USB port. The advantage of this option is that we have tested on everything from dialup to broadband and it works very well. CV guarantees QOS fr VOIP packets over the Internet for any speed, which is critical.
I actually perfer to use a Mitel or Avaya solution since it is much, much, much easier to setup (and it's included) than 3Com's. But we work with what we are given.
HTH