No how to for this, ATA 'S configuration re not standards so they change drastically depending of the brand, I have this same scenario but using a Grandstream FXO gateway and ZOIPER softphone on my Android phone
I think you are getting SIP user names and extension numbers confused. The latter are only defined for outgoing calls.
In general, you treat FXS ATAs exactly as though they were SIP phones, and FXO ones as though they were ITSPs, except that they may differ with regard to authentication and registration.
The short form is that there is nothing particularly special that goes beyond the basic knowledge you need to use Asterisk with an internet connected device. The forum is not a place to teach you that basic knowledge.
The Obitalk 110 is the cheapest 1 port FXO standalone unit you can find. It works with Asterisk and I have one on the NL landline network and have had no problems with it. This model has been discontinued, but you can still find them used (and sometimes new) on eBay. Lots of configuration guides, if you Google.