The documentation on the SigFox library for the endPacket() function contains an error. It says:
"Returns an int: 1 if the packet was sent successfully, 0 if there was an error"

while the example code says ( and is correct):

if (ret == 0)
 Serial.println("OK");
 else
 Serial.println("KO");

URL: https://www.arduino.cc/en/Reference/SigFoxEndPacket