Noise In Telephone Systems

I would change the hard drive cable to a shielded one.

Like this : http://www.ebay.com/itm/1m-3-Ft-SATA-II-SATA-to-SATA-300-Shielded-Hard-Drive-Data-Cable-for-external-/300946322221

JP, good suggestion, but if that doesn't work it may mean the drive is electrically noisy and the motherboard is re-radiating the noise through all the attached wiring, there's also the chance that the drive itself is poorly grounded or faulty.

OP, if you have a SSD (Solid State Disk) lying around you might try replacing the existing HD with it. SSDs draw much less current and don't have any pesky moving parts to cause spikes, plus they are much, much faster. Let us know what happens.

Does the EPABX have a ground connection ? phone systems operate on very small signals and induced noise is a problem for all low signal devices. Since unplugging any or all of the phones from the PABX does not solve the problem, the source is on the system side, not the phones themselves. you have discovered the noise is being generated on the DVR unit so how is that grounded with relation to the phone system ? if there is another connection between the dvr and phone system other than the common ground, you may have a ground loop via that path. Try to isolate the dvr from the phone system. does the phone system wiring share conduit with the camera wiring ? does the PABX chassis sit directly over or under the DVR in a rack ? If in a rack, you might have to isolate one chassis from the rack frame. You may have to add shielding between the PABX and DVR boxes. Are the cables for power and signal lines routed together or separately ? Lots of places to look.

What platform/framework does the PBX utilize? Is this a VME chassis or similar? Main board chassis frames utilize SMPS's and these are notorious for utilizing separate grounds internal to the power supply itself...DC reference. If the power supply is not properly shielded, the wiring for the output is not run properly, etc., noise can leak over to sensitive analog devices nearby or on the same ground plane.

I have had numerous issues with these power supplies in situations such as this. By isolating connections to the chassis, shielding, and referencing the low side of the power supply to the chassis ground (a VME option in most cases), and installing dedicated ground connections--liberal application of Kirchoff's law--I was able to remove all noise from my audio system.

Noise levels before my changes to the system exceeded 1 volt periodically, and I was able to reduce the noise level (sustained) to less than 10 millivolts.