If interference continues after trying filters, simplify the problem by unplugging all phone equipment from the jacks. Don't overlook spike suppressors and modems. One at a time, reconnect and listen. This can identify one or more defective or RF-prone devices that are the source of interference. Clean and tighten connections.
Interference can enter from the phone company drop.
Confirm this by disconnecting the customer side of the demark and monitoring the drop. If interference is present, they are responsible, but getting cooperation can be frustrating. Try to develop a relationship with someone on the technical side who can help you cut through the bureaucracy. WKOX's Andrews said that demonstrating ability to correctly diagnose problems yielded the direct number for the repair foreman. With business telephone systems, the best plan of attack is to filter incoming lines ahead of the switch.