HT-503

We transitioned to a cloud based Fusion PBX system from an onsite asterisk mypbx box.
We share office space with a client and had an old Linksys SPA400, 4 port FXO gateway connected to a FXS port on their legacy PBX which allowed them to call us as an extn on their pbx and we could call their extns and make external calls using their trunks. It worked really well but I couldn't get the SPA400 working as a gateway on FPBX so looked around for another option.
The Grandstream HT-503 was cheap and had 1 FXO and 1 FXS ports.
The FXS port is straight forward to set up and works well but we don't use that.
We set up the FXO port as an extn on FPBX (408) and we dial that number, get dial tone from their legacy pbx and then we can ring their extns or make external calls.
They call us on the same extn number they have always used and the HT-503 forwards the call to our ring group.
A cheap solution that works well for us.