?

My father is angry with his mother and since a few months even forbids me to send SMS to my grandparents. Can he do that?

Every child has the right to be in contact with his/her family members. Article 374 of the Civil Code repeats the International Children Rights Convention's principle:

The father and mother can't, without serious reason, prohibit the child to have personal relations with his/her grandparents. If the parties can't agree, the court arranges the modalities of those relations. 

In consideration of exceptional situations, the court can grant a correspondence or visiting right to anyone else, related or not.

You parents can be in conflict with their parents but they don't have the right to forbid you to stay in contact with your grandparents or other family members.