It appears that the only way to bring up audio volume control (as opposed to
ringer volume) on a C139 phone running Mot official fw is to be in active call,
implying the need for a valid SIM, a GSM network, working service with that
SIM+network combination and a party one can test-call.  When you do get into
the necessary active call state, navigation left and right buttons bring up the
audio volume control screen and move this volume up or down.  However, instead
of seeing just 5 possible volume levels as would be available if Iota ABB
downlink volume controls were used in TI's simple way, we see 8 possible volume
levels being offered by Motorola's fw, numbered from 0 through 7.  Level 0 is
not mute - downlink audio is still present in the earpiece speaker, albeit
quiet.

It appears that Compal did something similar to Foxconn/Pirelli, probably
manipulating both volume and PGA gain settings in Iota VBDCTRL in order to
offer more than 5 possible volume levels to the user.  However, because Compal
firmwares don't have ETM and their oabbr TM3 command is broken (returns
garbage), we don't have a way to just read out the register settings for each
volume level.  Furthermore, because the audio volume setting is so difficult to
get to (requiring not only a SIM, but also network registration and a call),
doing a tfc139 break-in for each possible volume level would be too much work.