Hi again and thanks for that information.
OK - from what you describe, it "sounds like" it may be some "electrical" interference - that is either getting into the phone LINE, or the AC power adapter - and is being "amplified" by the main base unit.
Now this may be ANY "electrical" device or appliance, such as an air conditioner turning on, or an overhead florescent light fixture (ballast really), or even some compact florescent light bulb, or the newer LED light bulbs.
If it happens at "regular times" (certain hours of the day), to troubleshoot this issue I would first MOVE the MAIN BASE UNIT to a different location, as sometimes just a few inches or feet can make a difference in the noise level. If you can TEST and notice a CHANGE - you are on the right track.
You can also try connecting the AC adapter for the main base unit to a power strip that has "surge suppression" and perhaps line noise filtering - to see if that makes a difference.
The DIFFERENCE and CHANGE is what we are looking for - to try and find this issue.
And by the way, to answer your ORIGINAL question about "can the frequency be changed", the answer is No. These systems actually work on a "range" of frequencies - at least between the main base unit and the handset(s). But "buzzing" - especially if it is happening with BOTH handsets, would be from the noise getting into the MAIN BASE UNIT, and then transmitted to the handsets. So that is where we need to concentrate our search for the noise.
Does that all make sense?
Please let me know if you can do ANY of these TESTS, and what you find when you do.
Thanks, ***** *****