Why am I unable to call a number?

You could not block a number being called from your mobile number. Because the mobile operator will not allow you to do it. It is a very serious security issue for any country. Actually, your acquaintances are not giving permission you to call to his or her number. So you could not make any call to this mobile number. It is a simple matter. Your acquaintance blocks your mobile number in its mobile phone. It is better to lodge a complaint to your local police station if this acquaintance is giving you a constant disturbance. Otherwise, just forget the mobile number of this person and delete it from your mobile contact list. Sometimes you may face some peculiar disturbances like this one. However, in any condition never share this person's mobile number with your friends because you may get into a trouble if you are not careful about it. Even it can exaggerate your problem also due to your callousness. Moreover, you can inquiry for this mobile number at the mobile service provider of that number as if you know the name of the mobile service provider. So, take this problem seriously and try to solve it as quickly as possible.