What to do when FaceTime isn't the default calling app?

I use two phone systems on my computer. One for work, and one for personal use. For personal use, I use FaceTime (using my phone through my computer). For work, I use an app called Nextiva. I would like FaceTime to be my default app for making calls. For example, When I click on a phone number on a website or tell Siri to call someone, it automatically makes the call through the FaceTime app and not Nextiva. There is a setting within FaceTime where you can dictate what app is automatically used by default to make calls, and I am constantly having to change this setting back to faceTime. I'm not sure how, but it keeps getting automatically changed back to Nextiva as the default. Sometimes I'll have to change it every couple of hours, and sometime It will be a couple of days before it automatically changes. I've tried going into my computer's privacy and security settings and removing Nextiva's access within settings like "automation" and "input monitoring," but this doesn't seem to have any effect. The only other security settings it appears under are "microphone" and "camera" (which makes sense because my mic is located within my camera). I leave access to this obviously so I can still make and receive work calls. Does anybody have any ideas on how to prevent Nextiva from being able to control my computer and change the default setting for which phone system gets used? I've never had this issue with zoom. I would like the default setting to be permanently FaceTime. I am working on a 2022 Mac Studio M1 Max chip running macOS Sequoia 15.5.

[Re-Titled by Moderator]