True, but he wants to increase the BT phone conversation volume, not increase the volume for the TTS voice ..... or are you suggesting he use TTSVoiceEditor to lower the navigation volume so that overall volume on the 1390 can remain at 100%? That could work as he uses it at 70% for nav voice, but as he has to use it at 90% now to hear BT phone that could then only increase by 10 or 11% overall. Also depends on the default setting in the particular voice i.e. if it's already at a low volume, which is unlikely i guess that it's set less than medium. Certainly worth a look.

EDIT: Checked Jack & Jill TTS2 for 1xxx. Default volume is 65 for both. Karen & Lee both 85. Daniel 80 and Emily 75. All with a possible range of 0 to 99. [Also not tested].