You keep returning to your original argument, that the OBi should work because it has a higher REN. The reality is, ancient mechanical bell phones that are perhaps 50-70 years old, have old, corroded, rotten internal "networks" (an inductor/capacitor) that may or may not still be functional. Add to that, house wiring that may be altering the impedance of the line. Yes, in theory, 5 REN is better than 3 REN. However, your real-world test, in your house,
with your phones, fails. There is no guarantee that every phone will ring as if in a laboratory test using fresh, new components.
I described, in my earlier post, a troubleshooting procedure to determine if one of those old phones is causing the problem or not, by systematically unplugging one phone at a time, and seeing if the ring behavior changes. If one phone causes the problem, but not the others, when all but one phone are connected, then that phone is a dud. You can power that one phone with the Grandstream and leave the rest on the OBi.