Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
There is no greater risk of failure with comcast (cable) vs frontier (copper). The primary difference is that one is a digital line, and the comcast gets converted to an analog connection through a modem/gateway. I'd be sure to have a good quality surge suppressor and powerline filter on the comcast gateway, for both power and the coax line, but otherwise there is no appreciable difference and it should not cause any harm to your system, though it is indeed an old model and will need eventual replacement with a new system, preferably a digital or voip based setup. I can recommend some options for you if you'd like.
Well you'd be looking at replacing the entire system, meaning you need a new pbx server, and new stations.
For a basic analog system, I'd recommend a panasonic 824 system: http://amzn.to/1nOb3wO
If you want to get into VOIP options, there are several options where you can rent/lease the equipment and everything runs through your internet connection, and you just pay a monthly fee for each extension, such as with 8x8.com for example.