Custom SIP Web/Softphone Integrated into Web Call Center Panel

We are a secretarial service. We answer the phones for our customers. We have created an Agent Panel where agents can answer calls on behalf of our customers. The Agent Panel has buttons (Dial, Answer, Hang Up, Hold, Park, Transfer, Conference, etc) We need these buttons in the Agent Panel to work so that the agent via headset can answer calls, dial, place callers on hold, park calls, transfer calls and do conferencing all from their desktop. We have looked at Zoiper Web [login to view URL] and VaxVoip [login to view URL] We are looking for a developer familiar with integrating web phones into a web application and ensuring that it is highly stable. You should have a license or be able to deploy this until we purchase a license. For consideration please link to any similar applications you have done and your experience.