Well, keep in mind this forum is run by NCH, a software company that writes telephony and audio related software, so this is not the place to get unbiased information about the different software packages available.
The thing is, while NCH's Axon is a suitable PBX for small-medium sized businesses, it is a 'work with what you got PBX' in the sense that it is closed software. Because of this, there is not a lot of room for third parties to come in and make modifications or fundamentally change how the PBX functions. Axon was designed more for single campus installations, and it would be really hard to use it (in my opinion) to provide several customers with a PBX service. Each customer's extensions and virtual lines would be bunched together in the configuration and I would imagine it would quickly become a mess and eventually possibly overload the system.
There is other open source PBX software out there that is almost designed exactly for what you need. It uses the same PBX base for each customer, but it partitions it in such a way that each customer appears to have complete control over his or her own PBX. For a good example of what this looks like in practice, try a free account with www.pbxes.com. This is more for what you need and is still an affordable solution (in some cases it might even be free to use). But as I said, I'm not comfortable giving out specific names of software packages.