The set of instructions you are reading is Obihai's own interpretation of the Google Voice signup process. It is neither up-to-date, nor entirely clear on why the steps are being taken.
The big picture:
Google Voice is NOT a free telephone company, despite Obihai's marketing material to the contrary. Google Voice is an inbound call forwarding service. You get an inbound Google Voice phone number, and then calls to that number will forward to up to six different linked phone numbers, and/or to Google Chat, and/or to Google Hangouts. OBi devices emulate Chat (XMPP) clients, so calls are actually being forwarded to the OBi via Chat.
In order to be eligible for, and to obtain your own inbound Google Voice phone number, you first must submit one of your own, personal mobile or land line telephone numbers. THAT is the number you need to add in Obihai's step 5. Ignore their instruction to use a VoIP service provider for that number. They were referring to a SP such as Anveo, Callcentric,
voip.ms, Phonepower, etc. You can no longer use a SIP VoIP phone number as an "admission ticket" to claim a GV number. You must submit either a mobile number from one of the big four US mobile carriers, or a traditional land line number from a carrier such as AT&T, Centurylink, Charter, Comcast, Frontier, or Verizon. VoIP numbers can no longer be used for eligibility, due to rampant fraud. The number you submit must not have been previously used to claim a different Google Voice number.
Also note that the screenshots on Obihai's page have recently (in the past month) been obsoleted by the new Google Voice web page user interface (UI). The pages shown on Obihai's instructions are from the old, "Legacy" Google Voice web UI, which can no longer be used to set up a new GV configuration.