Monitor & Troubleshooting PBXs - VOIP

Hey everyone, a bit of a preface. I stuck in the middle of an argument between our network admin, a PBX consultant, and our users. I'm needing some advice on how to at least identify the issue.

We use on-prem PBXs spread around the US in each of our call centers and one at our corporate office. Our corporate office often complains about static like noise and one-way communication. Our network admin says it's not his network

The PBX consultant says he's set up hundreds of PBXs for many different companies. That it's almost never the PBX box and always the FWs. He keeps saying that he's looking into the issue but cannot find anything.

Our Users don't care who's at fault. They just want it fixed. Namely the CFO and COO are starting to raise the issue because the frontline employees never get their problems fixed, it's also directly affecting the CFO. His office phone works only 50% of the time.

I'm in the middle of this because the network admin calls me his "phone guy" I'm his gatekeeper. I get chewed out by the users, then both the PBX and netadmin deny any issues, tell me to go swap their phone for another one. Won't take any calls from users. What's worse is that netadmin has our director convinced that is the solution. So he has managers buy-in/confidence

All we've been doing when anyone puts a ticket in is replacing their physical phone with the same type/model they've had. I know this isn't going to solve anything.

I keep reading about VOIP. I see that it has it's problems but there's got to be a way that users phone's work more than 50% of the time... right?

Does anyone have advice, resources, or tools I can use to help at least point to the actual problem? Point to it and yell fix this? My understanding of networking/VOIP is novice at best but I'm willing to learn/try. I'm so tired of being chewed out.

Thanks in advance for any help.