Usually, you will find that when talking about PBXes, they are classified in Class 4 and Class 5. So, obviously, your next question is: What is the difference between a Class 4 and a Class 5 PBX?  I will try to explain as clear as possible.

Class 5

A Class 5 PBX deals with end-user. It provides final-user services such as Voice-Mails, Conference rooms, IVR and many another kind of services that a phone user may need. This kind of switches deals with the user interaction (SIP endpoints such as Soft/Hard phones). So, in general terms, you can state that principal client of a Class 5 PBX is end-user.

It is very common to find them in the retail market.

Class 4

Class 4 PBX is a little different. They don't deal with end-user, but instead, they deal with other switches. If you are familiar with networking, you can see Class 4 switch as a router that is connected to other routers. Class 4 ones do not offer conference rooms, IVR or any other endpoint item; authentication is usually tied to IP rather than registration.

You can find this kind of switches on the wholesale market.

FusionPBX has a click-to-call application that it is not very easy to find, but it is not hidden at all. The click-to-call capability allows the linking between to endpoints from a http event. To be more clear, a perfect click-to-call example is the one that bank pages have on their pages, where you put your phone number, click the button and after a moment you get a call from the bank.

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.

You can pay using: Mastercard, Visa, American Express, JCB, Discover, and Diners Club.
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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.

Enjoy!