KXTa624 will act as PBX in your case. I think its has FXO ports which connects to the analog line from PSTN. So you can connect that with 2600 router with 4 port FXS card (2 module of 2 port fxs card) at each site. So the diagram will look like following
Exts.....KXTa624.FXO.....FXS.2600....IP.....2600.FXS......FXO..KXTa624.....ph exts