You can also get used Cisco VoIP ATAs for cheap from eBay (mine was $20) with 2 lines that can be programmed for internal calls and incoming/outgoing SIP, that'

You can also get used Cisco VoIP ATAs for cheap from eBay (mine was $20) with 2 lines that can be programmed for internal calls and incoming/outgoing SIP, that's what I did to play around with old modems and also set up 2 old-style phones to play phone with my kids.