I'm posting this in case anyone is searching for information about low call audio on the BT4600 cordless phone, as this has been (is still?) a very popular model.
We purchased three BT 4600 DECT phones and answerphone package ("Trio") for my wife's elderly mother a while ago. She died some months ago and we purchased two new BT4600 handsets to add to the base station to use in our home. We now find that the three original handsets (firmware XH08323_H5920) is buggy and handset call audio level can't be adjusted: the newer handsets with firmware XH08323_H5922 have been fixed - not only is the call audio level fine, but it can be adjusted whilst a call is in progress - and it seems that the three handsets with the defective older firmware can't be re-flashed with the new software.
My internet search for "bt 4600 phone low audio volume" brought up a number of responses but all of the relevant ones discussed exchange line levels, and none of them mentioned the firmware, hence this post. Just to make it clear, the two BT4600 phones that were bought a week or so ago do permit the call audio level to be adjusted "in call" but the three original ones don't, and the audio level is fine on all five phones when the call is on loudspeaker. Hence, this fault is one the things that were fixed in the firmware updates since the original three phones were manufactured.