Problem solved but not understood
Thanks for your responses.
After wasting all day trying things and asking people I tried to switch sim cards with a friend. My simcard worked perfectly in his cellphone and his simcard worked perfectly in mine. I decided to call the provider technical people again, after in the morning they comvinced me that the problem is in the telephone and it's not their responsibility. After explainig them the news, the nice representative told me to come to their lab and get a new simcard. I asked "Why?" and she said that sometimes that solves the problem.
I did that and the new simcard works OK. How come? Don't know.
Thanks anyway and see you next time.