Thanks for the offer of help, Stewart, but this particular problem doesn't need to be solved. I can always pick up the landline and call my friend, it's a local call.
The reason for the test is that we are starting up a WISP to compete with our local ISP, who offers a top speed of a miserable 768k. (We're lucky to get half of that most of the time.)
We want to offer phone service to our customers, so we're evaluating wholesale VOiP providers.
This test was valuable. Now we know one potential problem that we can ask our networking gurus to avoid when architecting our system. I have a feeling our customers will be wanting to call their neighbors and will expect to hear each other!
By the way no matter what we did with regard to opening ports or not, DMZ or not, both the OBI and the 2102 always worked perfectly, coexisting nicely on my LAN, except for this one problem (which never went away, even when the OBI was completely removed).