hi LIDERM, and welcome to the forum!
There is a section "New to OBi" where most of your questions have already been answered. It would also help if you scan
documentation (don't worry about the details of
how to implement certain things - just
what can you do with OBi).
For example, you can't "replace cable company phone service with Obi" - OBi is a device, not a service. Given that "people call actually GV numbers" you will probably want to replace cable company phone service with GV. Those may be details, but they are important for you to understand.
Some of your questions are not clear. For example, you have two lines and 4-line phones. Are only 2 lines on the phone in service?
What is your definition of "session"? If receptionist is talking to person A, and person B called, and receptionist put A on hold, and switched to B - is it one session or two? Traditional line came with call waiting.
You say you currently have 2 lines, but then you say you prefer 3 or more.
Thank a little bit about your questions, do some search and come back! Good luck