Is it true that internal forces can't move a body, if yes
Then how a mobile switched on viberation moves when a phone comes... :eek:

If internal force is greater than external forces, movement could result... how do people move if not by internal force when muschles are used? Perhaps I don't understand the question.

I'm not sure what you mean by internal force. Surely if you look at a different reference frame, any internal force is actually an external force.

I think the question is valid.

What makes the mobile phone ring? What effects a connection with the phone?

Electromagnetic forces switch something on in the handset.

The same principle causes vibration of the phone.

Again, I think the principle of internal forces not causing a body to move is not valid
In case of chemical reactions. After all, bombs do explode because of such reactions.

Furthermore, I believe that the principle of internal forces not causing motion is not a universal one. It is valid in case of statics/dynamics/mechanics, but may not apply in all circumstances.