We don't "officially" allow PBX systems because our unlimited plans are priced for a house/small business only and PBXes allow you to share a single account with many people.

With that said, we don't actively enforce that or hunt PBX users down.

That clause is just there to protect us in the event that we have someone abusing the service with extremely high usage (sharing it with lots of other people).

We have to have very broad and clear cut rules in place to use to protect us if needed.

As long as your usage is at a normal level, you are fine.