Large organizations use technology from vendors such as Cisco to provide an audio concall service.
For small and medium-sized organizations, the choices are more limited. Usually they have to use either a hosted service or a limited-functionality consumer VoIP service such as Skype.
The Sonexis ConferenceManager is an interesting option. This customer-premises box bridges both VoIP and PSTN, and also provides Web conferencing. A typical spend for 300 or 400 employees is around $90,000 to $100,000. Payback is normally within 6 to 12 months, due to lack of concall service fees and greatly reduced long-distance charges.