Softphone GUI

I am looking to develop a Web RTC, Desktop & Mobile APP that can support SIP VoIP Phone calls. We are a Telco service provider looking to build our own Softphone that meets our customers needs. The softphone should be able to support multiple SIP / Account registrations and allow the user to disposition a call at the end and schedule a call back using a basic calendar feature to remind them to call back the contact. What is a sofphone: "A softphone is a software program for making telephone calls over the Internet using a general purpose computer rather than dedicated hardware. The softphone can be installed on a piece of equipment such as a desktop, mobile device, or other computer and allows the user to place and receive calls without requiring an actual telephone set.[1] Often, a softphone is designed to behave like a traditional telephone, sometimes appearing as an image of a handset, with a display panel and buttons with which the user can interact. A softphone is usually used with a headset connected to the sound card of the PC or with a USB phone." SIP Communication protocols "To communicate, both end-points must support the same voice-over-IP protocol, and at least one common audio codec. Many service providers use the Session Initiation Protocol (SIP) standardized by the Internet Engineering Task Force (IETF). Skype, a popular service, uses proprietary protocols, and Google Talk leverages the Extensible Messaging and Presence Protocol (XMPP). Some softphones also support the Inter-Asterisk eXchange protocol (IAX), a protocol supported by the open-source software application Asterisk." Basic Fetures "A typical softphone has all standard telephony features (DND, Mute, DTMF, Flash, Hold, Transfer, call history, call outcome/disposition etc.) and often additional features typical for online messaging, such as user presence indication, video, wide-band audio. Softphones provide a variety of audio codecs, a typical minimum set is G.711 and G.729." This is a project we would like to kick off right away, there is more furures then what is listed above which will be briefed once a proposal has been accepted. We would like the developer to consider using open source softphone with the ablity to customize the fetures we would like to add. Otherise if they have experience in build this type of applications we would be open to a custom build where we would own the sorce code after the job is complete.