Does iPhone automatically choose wifi instead of data transfer netwok (cellular)?

I am in India and suddenly noticed that Cellular data has been on for a week. Roaming has been turned off all the time and I've been using wifi. But when I saw that My cellular data usage is 728 MB (from sending pictures and videos to Facebook, surfing and some sms messages) I got scared. Will this result in a huge bill? Has my iPhone been using the Cellular data network or wifi? Johanna

IPhone 6