1. Go here and install PlugPBX onto your SheevaPlug via SD card via ..... www.PlugPBX.org
2. Use an Ethernet based ATA (analog terminal Adaptor) which SIP interfacing to FXS or FXO ports. The Cisco/Linksys SPA-3102 is a great example, with both types of ports (interface to analog phones and interface to the analog phone company).
You can also find lots of vendors who make PRI/ISDN style 'black boxes' that will provide SIP interfaces via ethernet. Don't muck with USB ports and interfaces, ethernet is universal.
The 3102 product retails around $60-80 bucks online.
Its that easy