Neither the OP nor I said that the OBi is using WiFi to connect to the router. The OP reports that WiFi connection quality suffers, not just that call quality is poor. The OP posted:
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Our wifi performance started slowlly degrading (Netgear 7550) with devices continually dropping, etc. We replaced the modem and now the second one is doing the same thing.
There is nothing associated with an OBi 200 that can "hog" bandwidth. As you pointed out, VoIP calls use less than 100 kbps. So, there is no reason to focus on speed tests.
Which suggests: there is something causing WiFi communications to be degraded. Powering off and physically removing the OBi from the close vicinity would be the definitive test to prove or disprove if the OBi is the source of the degradation. We've learned from many years of end-user support, to not assume anything about the user's setup, nor their technical savvy. Whilst you and I may know that stacking equipment on top of, or underneath, a WiFi router is a bad idea, I can't assume that the OP knows that.
Chances are, that the OP has some or all devices using 2.4GHz WiFi, which is vulnerable to interference from nearby microwave ovens, old-school 2.4Ghz cordless phones, baby monitors, etc. The first thing to do is to eliminate possible sources of interference.