Try a factory reset.
From the service manual:
There is the ‘test mode’ of the handset to check whether its features and operations are OK or not.
Please press buttons in order as following : *, 4, 2, 6, 8, 5, #, ‘Send’.
After entering the ‘Test Mode’ and then select ‘9. Factory Reset’ menu. The ‘Factory Reset’ mode let the
current setting initialize the default setting. Acoustic parameters and gains are initialized depending on
‘Release Country’ setting. Refer to Figure 16.