CallerID.com

PhonePad incorporates support for Caller ID devices from CallerID.com.  

They sell a number of devices which support multiple phone lines.  PhonePad is compatible with any of the Ethernet devices (not the Serial Port versions).

The bare minimum device is the Basic model with Ethernet connectivity.  These are available in 2, 4 and 8 line configurations.

Images are copyright CallerID.com.