Synchronous communication, such as a voice call, has a very strong prerequisite: that both interlocutors are available at the same time for the conversation. Available, willing and ready to dedicate the necessary amount of attention required for the conversation. When people are face to face it is easy for both interlocutors to see and understand if the other person is available for a conversation. When people are distant this readiness for conversation must be inferred or negotiated. The fact of answering the phone provides a first indication of readiness, but often callers ask for confirmation by querying the whereabouts of the person called or even more directly by ensuring themselves that the receiver can talk now.
Many sophisticated social techniques have been developed to ensure readiness such as sending an sms first to ask if a call can be made, fixing a set hour for a call (eg the Saturday call from a distant grandmother, or the fixed appointment on skype). Similarly the apparently rude habit of checking the name of the caller on the mobile before answering, has often more to do with assessing ones own readiness, than with screening out someone. The receiver usually knows the caller and may guess the topic and therefore may be able to estimate if he has the time, the emotional or cognitive availability in that moment to address that conversation.
When a person calls another, they are aware of asking for attention, interrupting the other persons activities and this type of request is not done lightly or unconsciously of the social implications. Most people will not choose a synchronous channel to communicate with a person in a very different hierarchical position, such as their superiors or their teachers. Less intrusive channels will be preferred such as email.
Giving and asking for attention in communication is far from a neutral social behaviour and requires a sophisticated understanding of social norms and practices. The social practices regarding who we should give attention to first, for how long, how much attention we should request are highly complex and links between attention and power, attention and gender, attention and education have been extensively studied. Charles Derber (1979), for instance, argues that status relations, in general, are fundamentally about the distribution of attention-getting and attention-giving across the social hierarchy. Those of lower status are expected to give attention to others. Those of higher status are expected to demand and receive the attention of others. Thus, when either lower status individuals give attention or upper status individuals get attention, they confirm their status.
This issue is still far from resolved and the basic rules of power, hierarchy and availability are still being respected even after a decade of mobile phone usage. Most people do not call their bosses on their mobile unless instructed to do so to report back for instance, most people will not contact officials outside office hours and preferably on land lines that indicate they are calling an institution and not a person. Most people will call just the closest friends on their mobiles.
This means therefore that voice calls are being done either at well designated times where it is assumed that a person is available or to a very small range of intimate contacts. Well known, intimate connections are people with whom the issues of attention have been resolved either because the mutual schedule is so well known that calls are only made when the other is known to be available, or because the relationship is such that the strategies for attention giving and grabbing have been negotiated in the past or can be done so on the spot with no big risk for the relationship.
The pitfalls of negociating attention are without any doubt the main reason why the mobile phone is being used in its voice format, to call so few people.