[WLUG] asterisk or other linux pbx

Hi All, I’m considering a move from a traditional (and proprietary) PBX to something open-source that i can manage. I know very little about this, so i’m trying to understand if my goal is even possible. Given that our provider (i.e. gatekeeper) is Comcast, i worry they prevent running self-hosted VOIP boxes. Currently the office has 4 lines supplied by comcast. The comcast cable modem has 4 telephone lines going into a Panasonic PBX, which they dispatch the calls to the POTS phones. I want to replace the Panasonic PBX since its proprietary and requires a call into the phone company to change anything. I have very little knowledge in this area, so i’m wondering if anyone here has had experience in this area. Thanks! - brad