mobile

Hello guys :)...sorry if a similar question was already asked.
I have a problem with my phone. When i want to make a phone call the screen goes off. At first I thought that the phone switched itself off, but later I noticed that the credit was gone...:/ I happens even after the factory reset. It also happens with another dialer from google play...however, people can still call me and there is no problem...no problems with sms either.
I have 2.3.6 Gingerbread
Please help