The KX-TG6700 manual at: http://www.panasonic.ca/PDP/OperatingInstructions/kxtg6700-oi-eng.pdfhttp://www.panasonic.ca/PDP/OperatingInstructions/kxtg6700-oi-eng.pdf
For best performance
Base unit location/avoiding noise
The base unit and other compatible
Panasonic units use radio waves to
communicate with each other.
For maximum distance and noise-free
operation, we recommend the following:
LPlacing the product away from electrical
LPlacing the base unit in a HIGH and
CENTRAL location with no obstructions
such as walls.
LRaising the antennas vertically.
LIf you use the handset near a microwave
oven which is being used, noise may be
heard. Move away from the microwave
oven and closer to the base unit.
LIf you use the handset near another
cordless phone’s base unit, noise may
be heard. Move away from the other
cordless phone’s base unit and closer to
your base unit.
Have you considered all these points?
Is there a way you can move the base station to another location in your house to see if the problem get better? I don't beleive your base station is faulty, I suspect interference from something.
I don't think the 2.4GHz set is causing any issues, but you never know, try unplugged it for a while to investigate.
If you do determine that the base station is faulty there probably isn't many options to get it repair other than sending it to Panasonic. May be cheaper to buy a new base station to go with your extra handsets.