You can get a Docsis 3.0 modem at a decent price! Motorola and Zoom sell them. Don't rent a modem, its a pure old cash cow. They rent out old models.
I had RCN too and had problems with them sometimes too, especially Internet.
The best you can do to relieve this situation is to get a back up connection (ATT DSL - which is cheap now) or 4G (Wimax) from Clear.
Just make sure you get a dual wan router or if you get another wireless router for wi-fi connection make sure they do not overlap on the same channel. The non-overlaping channels are 1,6,11m (talking about g standard, dunno about n).
You can always go with a long twisted pair Ethernet cable
You probably don't have too if they have a voip adapter . The way it works is this:
cable modem--> voip adapter + phone --> gateway (router)--> pcs
you must likely use gateway with built-in voip, modem and router.
I only use cell phone and never used VOIP so I have no exact idea. Where I lived and live now coverage has been always good both inside and outside. Now with 3G microcells less people depend on voip or pots (traditional)