poor transmission kxta 624 x series

[430s]

this is a uk installation
we have a kxta 624 system suffering from poor transmission [the distant party complains they cannot hear whilst the kxta users can hear ok]
the latest attempt at clearing this tedious fault is for us to change ALL of the terminals. the reason for this is that they are all x series terminals [the part number has a suffix of an X indicating they are export versions?].
the supplier [we are the maintainer only] says that in the entire history of the world an x series terminal has never had such a fault.
Therefore, has anyone else had simaler problems with these vesion terminals?

 

[cwc3]

have you done test calls directly on the c.o. lines ?
i would not be so eager to point the problem
@ the system it self!

 

[430s]

these lines were proved at the nttap as being ok both way