Discuss possible ways of assigning these costs fairly among a number of productive departments. If points of communication eg computer terminals, phone and fax extensions are distributed between departments and not shared, it may be possible to log usage eg call details, session times etc.
Possible bases are floor area, number of people, departmental sales revenue etc.
When a compromise method is used, it is usually agreed between departments on the understanding that all accept that it is a practical approximation and would be reviewed on a regular basis by negotiation between departments.