You will need a cell phone which works on the t-mobile network. If your phone does, then you do not need to buy another. (or whatever cell network you use to port, see below)
Once the number has been ported out of the cell line, the cell line is not needed and cannot be used.
Comcast may close your account as soon as the port is initiated. But you will need to verify with them to make certain it happens. Any other services associated with the phone may be affected.
Note you may have other problems...
Google Voice is intended to forward calls to another phone number and they do not support the OBi, and Obihai has said it will quit working mid-May 2014. You could then get another phone number somewhere else (probably cheaper than Comcast) and have GV forward your calls to that number, but why then do you (or your father) wish to involve Google Voice at all?
In addition to possibly losing the current GV capability, the steps you outline are correct but not guaranteed.
Possible steps where your plan may fail:
4. While rare, t'mo may be unable to port your landline number. You may be able to check with their support to check in advance. You may be able to use tracphone or at&t gophone instead.
6. google may not be able to port your landline number, even if it ported OK to your cell carrier.
I chose NOT to port my number to google voice. Instead I ported it to Anveo (a VoIP provider) which cost $24 for the first year (plus $0.80/month so I can call 911 if I want).
