Nokia 5110 phone LCD panel connections -
https://serdisplib.sourceforge.net/ser/pcd8544.html

Using the connection diagram from the site above have traced the LCD panel and keypad tracks back to the pads that connect the front panel PCB to the motherboard.

The pads look like those for a 28 pin SMD IC with a 1mm pitch. Standard ribbon cable is too fat so cut some from an old ATA100 HDD cable.
Soldering required a bright light, magnifier and patience.

The keypad number section use the same 4 row x 3 column layout and numbers (inc. * and #) as the MMBasic keypad command.
The 4 keys in the row above it can be wired as column 4 and use the MMBasic 4 x 4 setup. All that is required is to bridge adjacent pads C4abd and C4c in the diagram below.

The LEDs for the screen and the keypad have driver transistors connected to the LED Ctrl pad so can be controlled directly by a PWM pin.

As the power button connects to ground it could be used as a reset button.