IP Based telephony solution can be developed in various solution as well as Telephony solution running on PSTN signaling which will be used the latest technology to apply with the VoIP environment such as

•  Auto Attendant System with Unified Messaging service
•  Interactive voice Response (IVR)
•  Value Added call services for voice and faxes
•  Calling Management system
•  IPBased Outbound/Inbound call center
•  Fax server solution
•  etc