We have started to see an interesting niche opening up where customers
want to have IP phones on their desks and an PSTN gateway only.
There are using phones that have most of the basic PBX fucntionality
(hold, transfer, conference, forward, DND, etc) built in. They
connect a number of lines to the PSTN gateway and that is their office
phone system.

The FXO gateway would fit this niche quite nicely I think. If you
just added the ability to register phones to it then this could make an
attractive bundle for a small office. It already has voicemail,
which I always wondered what that was included for. You would not
need any of the other PBX features at all.

As this customer grew they could be transitioned to a 4x or 16x as a seamless upgrade.