Why does my phone intermittently not ring when someone calls?

I use the RC mobile app on my Android phone. Sometimes when a person calls, the phone rings. Other times, nothing happens and I just get a voicemail. This has been a problem for me for 2 years now. It hasn't just happened in my current environment, but even when I was using the browser app (instead of the phone app) it would do this. How can I get some form of reliability out of this thing? Whenever I talk to support they just call the number and it's usually a day that it's working so they just shrug me off. I'm not sure what to do.