Looking into this issue a bit deeper, I discovered SIP is know to be problematic when used on an private IP network behind a NAT enabled firewall. There are apparently various work arounds including the use of a SIP proxy,etc. Looking directly at the configuration of my obhi using the build in web server (as you would a router, printer, etc), it looks like the obhi is using a SIP proxy (hosted by phonepower).
Some folks suggest giving the obhi a static IP address on the private network and then using port forwarding for various port ranges, etc. I settled on simply giving the device a static IP, subnet mask, gateway, dns, etc. We'll see if that makes any difference.
Another suggestion I read about was to simply take the router out of the loop to help isolate whether the router itself is the issue. For what it's worth, I am running an Asus router loaded with DD-WRT.
Update (May 7th, 2014): - Since configuring my Obhi 100 to use a static private IP address (2-3 weeks ago), I have not experienced any further service disconnects.