Today I have published in OKay's RPM repository RPMs for Kamailio 5.0.3. Kamailio is a very fast, reliable and flexible SIP (RFC3261) proxy server. Written entirely in C, Kamailio can handle thousands of calls per second even on low-performance hardware. A C Shell-like scripting language provides full control over the server's behavior. Its modular architecture allows only required functionality to be loaded. Among available features: IPv4, IPv6, digest authentication, accounting, CPL scripts, instant messaging, MySQL, Postgres and UNIXODBC support, NoSQL backends Redis, Cassandra, Redis, Memcached, radius authentication, record routing, SMS gateway, ENUM, UDP, TCP, TLS and SCTP, transaction and dialog module, OSP, statistics support, registrar and user location, SNMP, SIMPLE Presence, Lua, Perl, Python, Java and Mono programming interfaces, WebSocket support for WebRTC, IMS extensions, embedded XCAP server and MSRP relay, DNSSEC, gzip compression.
RPM's are available for Centos 6 and 7. And you can find it if you type yum search kamailio.
Twilio is one complete VoIP carriers these days. Among its SMS capabilities, it provides sip trunks as well. The good or bad thing about Twilio is that its SIP trunk only works with SSL/TLS. This will help you to warrant your confidentiality but it needs more work from you to make it work. I will talk how I did it.