Mitel releasaed this statement back in 2006 about this situation:
Under normal circumstances, the COS option (handset volume adjustment - saved) will keep the volume level the same between calls. We have 2 occasions in which the handset volume is reset.
1. If the handset volume is at max. and if the set type is dual mode, the volume is not saved on next call
2. Any type of sets will not keep handset volume on system reboot or set reset
For the first:
FCC regulations require that when a phone's volume is left at the maximum, it must be reset to the default level. The reason for this is, according to the FCC, the maximum volume is intended for hearing impaired individuals. For this reason the volume must be brought back down to the default volume setting. The sets default level is volume level 4. In order to keep the volume at the higher volume after hanging up the handset, you must ensure the user sets the phone volume to at least one level less than the maximum volume.
For the second:
It is important to note that the adjusted volume setting is retained by the phone between calls. However, if either the phone or the 3300 ICP resets, then the previously saved volume setting is lost and the set's volume will go back to the default value.
Note:
Also verify that the Set's COS has Handset Volume Adjustment Saved Enabled. This will have the same affect no matter where the user adjusts the handset/headset volume to. If disabled it will always default the volume back to the default level of 4.
Hope this helps.