If you don't need a solution with a traditional PSTN line you can consider the use of a PAP2 Linksys. It is capable of handling 2 VOIP lines.

Connect to LAN
Shown below is a connection diagram displaying a typical setup with one Voixium Internet phone line.

A) Using a telephone cable, connect your telephone to the PHONE 1 port of the Phone Adapter.
B) If you have a second Voixium line on your account, then connect another telephone or a fax machine to the PHONE 2 port of the Phone Adapter.
C) Connect the included Ethernet network cable to the ETHERNET port of the Phone Adapter. Connect the other end to one of your router's Ethernet ports.
D) Connect the included power adapter to the POWER port of the Phone Adapter. Connect the other end to an electrical outlet.
E) The Power, Ethernet, and Phone LEDs will be solidly lit when the Phone Adapter is ready for use. The installation of the Phone Adapter is complete. You may pick up the phone and make calls.

Taken from: voixium.com 

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