With latest image it should this easy to start use Pinephone beta edition as phone with Maemo Leste:
- put sdcard with Maemo Leste (image wrote with dd) in sdcard slot.
- start your phone (first boot sometimes have to start two times)
- write you SIM-card-pin
- Call with Sphone dialer icon or write SMS with Sphone New SM- icon (both apps comes with image)

btw. this answer was wrote with Pinephone Keyboard.