Internet drops when call comes in on cell

I've had Xfinity internet for years, recently switched to xfinity mobile approx 6 months ago. Within the past week, when calls come in on my cell phone, the Internet completely disconnects/resets? The light on the modem will turn red then flash green then go back to normal like nothing happened. Called xfinity, everything looks fine on their end, except that when I turned off the modem to troubleshoot customer service woman said she was still getting a signal (no idea how) and as soon as I unplugged the modem, the call dropped - she called right back, and says she doesn't understand why that happened as the phone and internet should not be affecting each other. Yet here I am. Nothing changed inside my home no new devices added no settings changed on either my phone or internet, etc. Can anyone help direct me to a potential cause and how to go about fixing it? Thank you!!!