SIPS URI Limitations

Using SIPS URI implies that TLS is used preferably hop-by-hop until the terminating UAs has the target resource. However, in reality, most VoIP service providers do not support end-to-end TLS connection because of complexity, especially when the call transports through different service networks that have different policies and capabilities. Some UAs in the request path may not support TLS, and even those UAs that do support TLS may be required to maintain a persistent TLS (TCP) connection that is not acceptable to other peers.

SIPS URI inherits the limitation of TLS, which runs over only TCP and creates a scalability issue as mentioned before.