A few things. No one is going to readily jump into spoon feeding you all the details on your questions, and you have a lot of broad ranging questions. Using an obi requires a knack for technology and a techno-friendly aptitude. You have to want to make it work. There's no way around that.
Have you looked at the tutorials here
http://www.obihai.com/tutorials-tips.html? In particular the one named "Port Your Number to Google Voiceâ„¢" and "How to Get Google Voice"? You should start there and then come back if you still have questions.
Based on what you describe as your usage needs, 30 one-minute calls, try a simpler approach, and get and use a free Google Voice number and simply use it from within your browser. No desk phones, no Obi devices needed. You'd make and receive calls from within Gmail within any browser. Obviously the browser must be open for this to work, when it's off line a received call would go to Google Voice voicemail.
Look at the Google Voice videos here:
https://support.google.com/voice/topic/1707989. You'd want the "GOOGLE NUMBER" account type, no charge to sing up, no charge for calls anywhere to the US and Canada.