Zendesk Talk is a ready to use and cloud-based VoIP system that's implemented in the Zendesk Support ticketing system. It allows customer service teams to provide phone support from the Zendesk platform.  

The LDAP (Lightweight Directory Access Protocol) is a vendor-neutral, open, industry standard application protocol for maintaining and accessing distributed directory information services over an IP (Internet Protocol) network. LDAP enables access to locate organizations, individuals, and resources such as files and devices in a network, both on public Internet and on corporate intranet.