Quote...despite being hardwired to a MESH extender/router...
Please provide more details. Do you mean: you have one main router, and the extenders are linked to that router via WiFi (hence your use of the term "mesh"), or is there an Ethernet cable running from the main router to each extender? By "hardwired", do you mean that you connected an Ethernet cable from the extender to the OBi device?
If my assumptions are correct, then periodic, brief signal quality issues in the WiFi link are causing the problem. If this is a typical cable ISP service, then you likely have asymmetric service, with say, 50Mbps download and a much slower upload. It's possible that some other device on your premises network is periodically maxing out the upload bandwidth, which will cause the party on the other end of your call to experience drop-outs or distortion in your voice.
The next step would be to move the OBi device over near the main router, and connect the router and the OBi via an Ethernet cable. That would narrow down the problem. If performance improves, you have a WiFi issue; if it still fails, you have an ISP bandwidth issue.