Asterisk

Pros

It's hard to know where to start with this. It completely changed the telecom space by breaking away from the control of major players like Nortel. It delivered the same level of reliability while also bringing IP technology into the picture. Rather than just competing with the old model, it reshaped it by giving companies of every size a flexible, open source option with real power behind it.

Cons

That said, Asterisk can be tough for the average person because it's deeply technical by nature. In the beginning especially, everything needed to be set up by hand which made it far less approachable for non technical users. Still, when it comes to what it can actually do, it's hard to fault. There's a reason people call it the "Swiss Blade", it's adaptable, powerful and able to take on nearly anything you put in front of it.

Pros

There's no cost to use it, it follows an open standard and it can be fully customized. AGI scripts make it possible to add even more features and connecting this with other software is very simple.

Cons

Honestly, I'm happy with everything in this. It really feels like the best option.

Pros

Running this alongside FreePBX for our business phone system has delivered a lot of value although getting it deployed and configured is definitely not a simple job. The documentation is extensive but since it's an open source project, it still takes some trial and error to fine tune everything to the best possible setup. From a cost perspective though, it's extremely economical and that's a big reason I see it as a top tier option worth considering for businesses of any size. We currently have about 300 phones deployed on Asterisk and FreePBX and compared with the pricing of major phone system vendors, we're saving tens of thousands of pounds every year. It's not perfect but it offers a very rich feature set and if you're prepared to invest the time and learn it properly, you can build your own applications or custom scripts to shape the system around your business needs exactly the way you want.

Cons

Deployment is where the challenge really shows because this is not the kind of system every IT engineer can set up without prior experience. It takes time, patience and you should expect to hit a few gremlins along the way but even with that, overlooking it would be a mistake.