Cisco

has a full suite of IP communications solutions and endpoints that let you extend consistent communications services to employees in all workspaces, whether they are on a main campus, at branch offices, or working remotely.

Cisco call-processing solutions have received stellar marks from users who give high ratings to voice quality, network integration, and interoperability.Call processing solutions include:
Cisco Unified Communications Manager: This centralized, server-based solution provides enterprise-class IP telephony call-processing. It supports up to 30,000 users per cluster and is ideal for mid-sized businesses and large enterprises.
Cisco Unified Communications Manager Session Management Edition: This version of Cisco Unified Communications Manager serves as an aggregation server, pulling together devices and applications. It supports up to 20,000 concurrent sessions and is ideal for mid- and large-size organizations.
Cisco Unified Communications Manager Business Edition: This solution contains voice, messaging, mobility, and video capabilities integrated as an all in one system on a single server.
It offers low total cost of ownership for medium-sized businesses with up to 500 users and 20 sites.
Cisco Unified Communications Manager Express: This solution provides call processing for Cisco Unified IP Phones as part of a converged voice and data solution empowered by a Cisco router.
It supports up to 300 users for single site small businesses or distributed enterprise branch offices.
Cisco Intercompany Media Engine: This solution provides a revolutionary way for organizations to extend their Cisco Unified Communications experience to their business partners or customers.
It supports up to 20,000 users per server and is ideal for all size organizations.
Cisco Unified Survivable Remote Site Telephony (SRST): This solution provides router-based, multi-feature call processing redundancy for centralized Unified Communications Manager and Unified Communications Manager Business Edition deployments. It supports up to 1200 users and is ideal for branch offices and remote site teleworkers.
Cisco Smart Business Communications System: All-in-one voice, data, video, and security solution can be centralized for small businesses or distributed in branch offices