You cannot pass POTS (Plain Old Telephone System, AKA analogue phone system) down a TCP/IP network so you won't be able to use analogue phones with a PowerLine system.
Indeed, PowerLine systems only take a single master Ethernet feed from your router so you would need two separate systems if you had even a digital PBX.
Instead, the way to do it is to use VoIP. For that, you will need a system that takes POTS as an input and converts it to VoIP and visa versa. Then your data and voice can both occupy the same Ethernet cable.