Hi mate,
The cube doesn't work alone, most of the time we install the emergency phone on top of car, under the inspection box or next to it ( 3300 )
The only way to change the phone number will be on the emergency phone, not on the cube.
To access on cube you must use a shindler app, call IoEE - SetupApp.
I highly recommend to install a new phone with physical line or gsm.
Cheers