Query related to PBX and Conference

Scenario: There is a conferencing service offered. Each telephone line is considered to be one account. The service basically is to call a number of the service provider. Add groups(containing our contacts). So the problem comes when a customer is calling from a PBX extension line. So that customer can access all contacts ie contacts from other extensions.Any idea how to avoid this problem. the solution to have different identities for PBX extension lines.