The One Ring uses the glib mainloop. As long as its not responding to anything, it is completely off the processor. The times it responds to something besides direct user interaction is all configurable including doing nothing. At peak on my desktop I got it at 0.6% CPU

As for memory that could be an issue. On my desktop it peaked at 0.7% memory.

As for storage, I think new installations of python are optified so if you didn't have python on it before, it shouldn't take up rootfs. Come on, 2GB is plenty of space for applications

Adding Google Voice contacts is expected behavior. It can be disabled in the "Advanced" settings. DO note that currently TOR doesn't support System contacts so if you do not have GV contacts enabled, you have to hand enter each contact.

As for placing calls not working, this depends. Could you not enter numbers and click dial in the phone app? Did it give a general error? Did it just report "disconnected"?

The first two questions are bad things of "did not allow". The last means it worked! GV works by using a callback number. So you "place the call" which immediately disconnects, GV will then call your callback number, and then GV will call the other person. If if reported "Disconnected" and didn't try and call you,I recommend checking the callback number you have configured. It should be displayed in your "Screen Name" field in the account settings.

I recommend reading the first post in this thread, it has a lot of good info, including the stuff I'm currently repeating (I think).