Voice calls to India disconnect every 15 minutes

I recently signed up for Xfinity Voice. I have my own router and modem. Signals are very good. Strangely, voice calls to India disconnect consistently every 15 minutes eventhough when i receive calls, i am able to speak at length. I had Vonage voice service 10 years and voice calls would disconnect every 60 minutes. I had a different set of modem/router equipment also owned by me. Only common thread is the Xfinity internet service.