Seeking to understand the 16-pin interface between the main PCBA and the keypad.
Both connectors (the one on the main PCBA and the one on the underside of the
keypad) have silkscreen with pin numbers around them, but the numbers in these
silkscreen labels don't match: per the main PCB silkscreen, pin 1 is toward the
edge of the phone, whereas per the keypad underside silkscreen, pin 1 is toward
the middle.

Here I will use pin numbers per the main PCBA silkscreen.

Geometric centre of the footprint is at (3923,456) in steve-m's L8 scan.

Pin 8: connected to massive copper flood, almost certainly GND.

Pin 9: connected to a copper fill that's islanded by some traces, so it may or
may not connect the same massive fill (GND?) as pin 8.  In this island there is
a largish solid-connect via at (3915,475) and two smaller ones at (3766,539)
and (3911,507).  On L7 these vias connect to massive copper flood - definitely
GND.

The keymap given in OsmocomBB (confirmed to be correct) indicates that the main
keypad ought to use KBC0 through KBC3 and all 5 KBR lines.  The 3 side buttons
use KBC4 and KBR1 through KBR3.  Let's try tracing the KBC and KBR lines of
interest from the Calypso.

Tracing KBC0, Calypso ball N4: goes to via at (4122,380).  Found it on L2, goes
to a larger via at (4130,444).  Found it on L7, trace disappears into the edge
grind-down damage.