i have a USB/Sip phone - Plantronic Calisto P240 - and I am curious to know if I could use it with my Obi200. If I connect it to the USB port, shall it work as a regular phone?
No.
USB phones are typically little more than a USB sound card with mic and earphone plus a keypad and sometimes a display. Everything except the analogue to digital and digital to analogue audio conversions has to be done on the host PC.
The USB socket on OBi devices does not support them.