Comprehensive VOIP Services Implementation

I'm looking for a skilled professional to help implement a comprehensive VoIP system in my organization.

Key Requirements:
- Voice Calling: This will be a primary function of the system, so proficiency in voice call setup and management is crucial.
- Video Calling: We'd like to incorporate video calling capabilities for select users, improving the quality of communication.
- Instant Messaging: Our system needs a robust instant messaging feature to facilitate quick and efficient internal communication.

Additional Requirements:
- Scalability: The system should be designed to grow with our needs, accommodating a potential increase in users and usage.
- Security: The utmost importance must be placed on security measures, ensuring our communications are private and protected at all times.

Please let me know of any prior VoIP implementations you've done, and how you plan to address the scalability and security concerns. Your bid should also include an estimate of the time needed and the budget.