What does it mean?
Appelle le 17.
Translation: call the 17
is it police?
Yes, it's the police ( in French "police" or "gendarmes"). 15 for an ambulance, 18 for the fire service, though they often deal with medical emergencies too. It all goes through the same centre where I live, so it doesn't really matter much which number you call!
Yes, that's right, but be aware that 911 only works if you have a phone set up to redirect the call to 112 (the Europe-wide emergency number, which, as you say, also works in France).