SPA3102 is no longer in production (although refurbished devices are still available from some sellers). The only Cisco's SMB ATA with FXO port is SPA8800 which is so expensive for the purpose.
Thus, either buy refurbished SPA3102 or search for equipment from other vendor.
But I feel responsible to warn you - there are issues related to the matter of your idea. Those issues will be here regardless the exact product and model you will use.
Analog (POTS) line has not been designed for automatic service.
Imagine call from PABX side to (remote) phone. There's no standard way to signal the calling party has ended the call. Moreover, established call can't be terminated from PABX side of analog line. The ATA device must "hang up", but it doesn't know when it should do it.
There are some heuristics method related to id - silence detection, a kind of CPC signal issued by PABX (if it support it *AND* if PABX itself know the call has been terminated by caller) - but it's rather heuristic, not reliable method.
To make things even worse, there's nothing like "world standard for POTS line signalling". Voltage levels, impedance required, ringing signals, dialing method - all those are specific to particular model of PABX. Although ATA connected to PABX will try to fit PABX requirements, it' may fail. In such case, ATA will not understand PABX signals and vice versa - no call will be possible in one direction.or in both directions. Debugging of those issues require at least voltmeter, sometime oscilloscope - and skills.
Despite your goal can described by simple sentence there are so complex issues hidden beyond it.
There's so thin border between "it will work, although unreliably" and "it will not work at all" even at hardware level of your design. And no one remote will tell you for sure "do it this way, it will work" (unless he tried it on same PABX model including the firmware version and configuration).
I'm still speaking about hardware issues. Software configuration is next part of the show.
In short - unless you understand the principles of POTS analog phone line and understands at least basics of SIP protocol, you have so high chance to fail. SO, my best advice - don't design something like it by self, unless it's just challenge and you don't care the results (as well as money spent).
Well, I feel not so helpful now.
There are users trying to do something like you. Read:
Unless their experience will make you optimistic, you should consider to buy professional POTS line extender instead - like Ethernet Telephone/Analog Extender (TC1910) or so. But I assume it will be expensive a lot, moreover, some inter-operability issues apply even for professional solutions.
Other solutions ? If you PABX can speak SIP (or it can be extended to do so), forget analog line at all, connect SIP phone on remote side by SIP.
Hope something written above will help you to make decision ...
