I don’t know why the variable is correctly updated within that if block, and the result is visible using a print, but outside the variable nothing seems to happen. I suppose that the issue is related to scopes, since it seems last_dialing_number is created only locally.

def serial_digest(lock):
    ...
    last_dialing_number = 0
    with GSM(lock=lock) as g:
        while True:
            if g.data_not_available():
                continue
            data = g.receive()
            call_status = ''
            if data == 'RING':
                call_status='Ring'
                g.transceive('AT+CLCCrn')
                data = g.receive()
                if data.startswith('+CLCC'):
                    for x in data[len('+CLCC: '):].split(','):
                        if x.startswith('"') and x.endswith('"'):
                            print(last_dialing_number) # correct value
                            last_dialing_number = x.strip('"')
                print(last_dialing_number) # incorrect value