Quality of voice - As IP was designed for carrying data, so it does not provide real time guarantees but only provides best effort service. For voice communications over IP to become acceptable to the users, the packet delay and jitter needs to be less than a threshold value.
Interoperability - In a public network environment, products from different vendors need to operate with each other for voice over IP is to become common among users.
Security - Encryption (such as SSL) and tunneling (L2TP) technologies are developed to protect VoIP signaling and bear traffic.
Integration with Public Switched Telephone Network (PSTN) - While Internet telephony is being introduced, it will need to work in conjunction with PSTN in the foreseeable future. Gateway technologies are developed to bridge the two networks.
Scalability - VoIP systems needs to be flexible enough to grow to large user market for both private and public services. Many network management, user management technologies and products are developed to address the issue.
