when the telephone first had to have a modem
This is when Cox changed their telephone service to VoIP. Cox Digital Telephone (old service) was circuit-switching; VoIP is packet-switching.
The tech said it was an outdated modem
An eMTA is the telephone modem. The DOCSIS portion of your DPQ3212 is DOCSIS 3.0. The newest version of DOCSIS is version 3.1. Yes, the DOCSIS-side of your DPQ3212 is "outdated"; however, you only have a 50 Mbps plan. I still use a DOCSIS 3.0 because it has been more reliable and online since I got rid of the Cox-issued DOCSIS 3.1. I haven't had a problem with connectivity since I installed it.
Cox did not provide service to a voice modem that was not issued by Cox
Again, you can't buy a telephone modem at Walmart and connect it. Cox must issue the telephone modem. If you're getting telephone service, there is nothing wrong with the telephone-side of your DPQ3212. As Mouth recommended, just go freely exchange it. Don't get duped into renting their 3-way combo device (WiFi Panoramic). You don't need it.
WiFi extenders connect to your router...not the modem. Look for extenders compatible with your Nighthawk. The only thing you need from Cox is the telephone modem. That's it. Stop shopping for Cox-compatible stuff.
I don't think there is anything wrong with your equipment. It's just the service to your house. However, you can rule-out your DOCSIS by freely exchanging it.
Wrong you can buy cox will not activate it
