It is possible that the other driver did not make the call because he knew there was something wrong with himself or his vehicle. The police would normally attend in the case of injuries, or an allegation of a criminal offence, e.g. careless driving, or in the case of serious damage. They would also attend if the road was blocked etc. In an ideal world they would attend to see whether these circumstances existed. We do not live in an ideal world, and they will often find a reason for not coming.