Despite your assertion that you've had your hardware for years, without changing it, this sounds like a router issue to me. Unless you knew what you were doing, you may have chained the two routers together, creating a double NAT setup. You either need to configure the TG to run in bridge mode, or move the OBi's Ethernet connection to the TG, not the Netgear.
As Taoman already pointed out, the definitive way to prove or disprove this is to temporarily take the OBi and telephone to some other location's network and see if the symptom changes.