It's not as bad as some of the other commentators say it is, PROVIDED you build Asterisk and FreePBX yourself on a distribution such as Debian, although it is certainly not bug-free (and they are very slow to fix bugs sometimes, and sometimes deny that a bug exists because they can't replicate it or don't understand it). FreePBX absolutely will not serve you well in that situation. Multi-Tenant has many meanings, but when you clarify it with 'We want the option to offer PBX service to other business customers' then I would say maybe you should take the training and use FusionPBX. Now, what the developer would REALLY like you to do is attend one of his training $eminar$, which are VERY expensive for the small user, but in your case it might be worth it.

Seriously, FusionPBX is probably a great program until you run into trouble. I LOATHE IRC for many reasons maybe you don't and would find that an acceptable way to get support, but I don't. And their idea of support is that you go into an IRC channel, post your question, and HOPE someone knowledgeable just happens to be hanging out there and will respond.

But beyond that, I found out that once I got it running, things just didn't work right. This is very true of the FusionPBX documentation. Yes, they know their software better than anyone else, but that's the problem - they make assumptions about what you know, and usually you don't know what they think you know. I am of the firm opinion that developers should never write their own documentation. You will find lots of videos, web pages, and books devoted to FreePBX (including how to get around its quirks), though frankly speaking a lot of them contain bullshit information that will get you in more trouble than if you'd never read them.

But the thing I found about FusionPBX is this: There is an abysmal lack of good, free, understandable documentation about it. I have been using Asterisk and FreePBX ever since it was known as and there are many things I don't like about it, but many things I do. I have tried to use FusionPBX two or three times now, most recently this summer.

Signalwire freepbx