Want to drop Xfinity modem for an owned modem

I have GB internet service, but I also need to keep phone services. What is the best option for cable modem with voice and can handle the fastest speeds? The phone thing seems to be the achilies heel. Our cell phones dont work at the house very well and we like to have home phone so it can absorb telemarketers, unwanted calls, and just the fact that the cells do not work great here. I tried to switch to Netgear C7000 and went to a Voip seperately from Xfinity. The C7000 must have something internally wrong because it was having connection issues. A Comcast tech came out and had same issue, so maybe the modem is bad, or the port. I have looked and seems only real options is Motorola MT7711 or Netgear 7100V but Id like to know if these are any better (faster, more stable, etc) than the Technicolor gateway Xfinity provided. Total novice with this stuff, so be nice.