What should I look for when eliminating static on a VoIP line?

A small organization that I belong to has VoIP service, with 2 phone numbers. The VoIP service connects to a Motorola cable modem, and the modem connects to a Linksys router. Then, they connected an IP-PBX to that Linksys router. Then, all 3 of their phones connect to the IP-PBX with RJ11 connectors. But you can hear a lot of static on every phone. I volunteered to fix the problem. What specific things should I look for with the cabling or the configuration of the PBX, when fixing this problem? Please give me as much detail as possible. I do not know the model number of the PBX.