Cisco

IP telephony provides a way for you to extend highly secure, reliable, and consistent communications services to all your employees whether they are in main campus locations, at branch offices, working remotely, or are mobile. IP telephony transmits voice communications over the network using open-standards-based Internet Protocol.
Cisco IP telephony solutions are an integral part of Cisco Unified Communications, which unify voice, video, data, and mobile applications on fixed and mobile networks enabling users to easily communicate in any workspace using any media, device, or operating system. Using the network as the platform, Cisco IP telephony solutions help organizations of all sizes realize greater security, resilience, and scalability in addition to the inherent benefits of using a converged network for transport and interconnection. You can use Cisco IP telephony solutions to:

Provide highly secure, reliable, scalable communications that take advantage of your LAN and WAN
Improve employee agility and productivity through integration with innovative Cisco Unified Communications and third-party applications.
Cisco IP telephony solutions comprise two categories:

Call processing
IP phones
Wired
Wireless
Softphones
Cisco Unified Communications call processing solutions range from Cisco Smart Business Communications System (SBCS) to Cisco Unified Communications Manager Express for small to medium-sized businesses (SMBs) and enterprise branch applications, to Cisco Unified Communications Manager Business Edition and Cisco Unified Communications Manager for larger medium-sized business and enterprise applications. These call processing systems provide voice, video, mobility and presence services to IP phones, media processing devices, VoIP gateways, mobile devices, and multimedia applications. As Cisco Partners, MyISP have a huge range of products that cater for all sizes of business that vary through the ranges.